What is Haunting Melody?
Haunting Melody is a [General] feat; no special rules govern them as a group.
The Book of Exalted Deeds says that These feats are thus supernatural in nature (rather than being extraordinary abilities, as most feats are) (BoED page 39) concerning Exalted Feats. This strongly implies that all feats are extraordinary abilities, but there may be table variance.
As for extraordinary abilities,
Using an extraordinary ability is usually not an action because most extraordinary abilities automatically happen in a reactive fashion. Those extraordinary abilities that are actions are standard actions unless otherwise noted.
Thus, if one's Dungeon Master allowed them to use Haunting Melody on its own, it would be a standard action.
But Wait, There's More!
While using Haunting Melody "counts as one of your daily uses of bardic music," it is not a "bardic music effect"—those are those abilities listed under "Bardic Music" with their names italicized. Unlike those effects, Haunting Melody may be used within an Antimagic Field, as it is a [General] feat and Bardic Music itself is not listed as a Supernatural or Spell-Like Ability.
Now, see the link to the description of an Extraordinary Ability up there? According to the text of Haunting Melody, the feat is a reactive ability with the triggering condition "When you sing or use some other Perform skill". Thus, whenever you meet the triggering condition you may opt to expend one usage of bardic music in order to 'inspire fear in your enemies'. Nothing in the feat description states or even implies that this is a standard action. If you can use Perform as a move-equivalent action, for example, the Gladiator's Roar of The Crowd (Sword and Fist page 21), you may use Haunting Melody then. If you can use Perform as part of casting a spell, as Music of Making implies is possible (but does not explicitly grant the ability to do so), you may use Haunting Melody then. In fact, if you possess Melodic Casting (Complete Mage page 44) or Chord of Distraction (Complete Scoundrel p. 75), you may even use Haunting Melody off-turn! Dragonsong (Draconomicon p. 105) increases the saving throw by 2, and there are many other feats and spells it can be used as a part of.
Too Long? Didn't Read? No Worries!
In summary:
- Haunting Melody is a reactive, extraordinary ability.
- It is triggered "when you sing or use some other perform skill"
- It can be triggered at any time, even during another character's turn, as long as the triggering condition is met
- It can be triggered when casting any bard spell (see the Players Handbook, page 26)
- It expends one use of bardic music upon activation
- Your DM may houserule to allow it to be used as a Standard Action
The special ability soothing voice is a neat but limited trick
The 1st-level half-elf bard racial substitution level replaces countersong with soothing voice (Races of Destiny 157), which is like the bardic song fascinate but different, so below I've inserted soothing voice's differences into the description of the typical bard's fascinate special ability:
A half-elf bard with 3 or more ranks in the Diplomacy skill can use his voice to cause one creature to be affected by a calm emotions effect. The creature to be affected must be within 30 feet, able to hear the bard, must be able to understand the language the bard is speaking, and able to pay attention to him. The bard must also be able to see the creature. The distraction of a nearby combat or other dangers prevents the ability from working. For every three levels a bard attains beyond 1st, he can target one additional creature with a single use of this ability.
To use the ability, a bard makes a Diplomacy check. His check result is the DC for the affected creature’s Will save against the effect. If a creature’s saving throw succeeds, the bard cannot attempt to fascinate that creature again for 24 hours. If its saving throw fails, the creature is affected as if by the spell calm emotions [Note: Several omitted sentences appear here that the DM could, potentially, apply also to the special ability soothing voice, but I have assumed here they are, instead, relevant to the special ability fascinate. Ask the DM.]
This effect is an enchantment (compulsion), mind-affecting ability.
While the spell calm emotions is usually is a 2nd-level spell (therefore usually not available until at least character level 3 and higher than that for a bard), its usual range is medium, its usual area is a 20-ft.-radius spread, and its usual duration is concentration (up to 1 round/level). Although the special ability soothing voice's save DC will be higher than the spell—based as it is on a Diplomacy skill check—, a low-level character being able to use a few times per day this special ability (that draws its uses from the same pool as inspire courage, by the way) isn't overpowered, especially considering that the bard must have languages sufficient to be able to make himself understood to the creature (unlike the song fascinate) and that affected creature can just walk away if the bard's uninteresting (which is insulting and hilarious).
At higher levels, soothing voice is probably a waste of the bard's time, with him having much better things to do with his bardic music ability and him having spells that will do what he was trying to do with soothing voice better than soothing voice, but soothing voice may yet even then give the bard the opportunity to use his real Diplomacy skill, a skill that can wreck the game in the wrong hands. That is, at least, until higher levels in which the vast majority of creatures are immune to mind-affecting effects (you know, like the special ability soothing voice).
Note that spell calm emotions neither erases memories nor changes attitudes. If Baracs strolls into the orc camp, murders the orc warlord, is caught red-handed by the warlord's lover, uses soothing voice on the warlord's lover, and the lover fails the saving throw, the lover will (greatsword in hand) calmly discuss with Baracs his actions and impending demise as he escorts Baracs to a cell. The warlord's lover, even while affected by the spell calm emotions, can still call for help, alerting the camp, and Baracs just won't have enough soothing voices for the whole tribe. And the minute Baracs tries something aggressive (and a DM may rule that a murderer caught red-handed who then tries to flee a crime scene is behaving aggressively), all bets are off, and swords start swinging.
The spell calm emotions gives the caster a brief respite in which to catch his breath and perform some minor, non-aggressive actions, most often the spell offering an opportunity to for the party to enter into an otherwise implausible parley. It does shut down melee attackers, but while the effect lasts it also essentially shuts down the party. (That is, "The distraction of a nearby combat or other dangers prevents the ability from working.") Unless the party's worked out tactics beforehand for ambushing becalmed monsters (which is at least rude if not outright evil), the only thing a calm emotions effect will really do is let the user talk to one or more creatures that were, moments before, trying to kill him.
I can imagine that might be overpowered in some campaigns, but I suspect those campaigns probably wouldn't allow such a special ability to be taken in the first place.
Best Answer
Yes, by RAW you can.
As per SRD, while starting bardic music is always a standard action, maintaining it is another story. Bardic music abilities requiring concentration to maintain explicitly say so, for example (emphasis mine),
Therefore, whether you can do something meaningful while maintaining Inspire Courage is up to the interpretation of the duration clause (why don't you just resort to these 5 rounds, or 1 minute?).
The description of the perform skill does not shed any light at this problem either (emphasis mine):
To sum up,
there is no action stated in the rules to maintain the Inspire Courage, so we can assume it is a free action. Therefore you're free to do whatever you want while singing (reciting, in your case), as long as you have requisite numbers of hands free (you probably do) and it is not prohibited by bardic music rules explicitly (cast spells, activate magic items by spell completion (such as scrolls), spell trigger (such as wands), or command word).
As a note,
the concentration on the bardic music is akin to the concentration on a spell; it even prohibits other casting. You could ask your DM if Extraordinary Concentration could be applied to the bardic music, too. Swift Concentration could also be useful for you, since it is already applicable.