When a bigger hammer doesn't suffice, intelligent heroes need to use Guile and Cunning... which sometimes includes talking enemies to death.
The concept of the "face" in role playing games exists because of some problems that cannot simply be solved by beating them to death with a bigger hammer. These problems include: getting paid, finding a gig, explaining that "it wasn't us" to outraged authorities, and all sorts of other... politics.
The need for a face, for a bard, is directly proportional to how much political wrangling the party faces. If getting paid is a matter of dropping a head on a desk somewhere and saying "gimmie my money" and... they do, then there is no need for a bard. If there's a entrenched bureaucracy between your (carried) head and that desk; suddenly there is a need for a bard.
Incorporate political problems and consequent bonuses to make playing the face appealing.
Players should be able to earn greater bonuses, avoid some combats, get assistance, and get paid because of the abilities of the face. So long as the group is willing, this can also include scouting duties and other unsavoury "we don't have a big enough hammer to solve this problem" problems. However, since some people play RPGs to get away from politics, this is absolutely something that must be discussed with the group first.
Stun, Daze and similar stuff don't impede your movement...
...they make your mind funny.
While you are stunned, there's nothing holding your body to move. You just can't think right to actually do something. If you take a really hard blow to your head, that won't make your body harder to move - it will make your brain go gonzo for a few secs, before you became aware of what's really happening.
Stun is not about movement, is about senses of what's going on.
Freedom of Movement makes "your body work right", not anything else. It allows you to move, but not allows you to think. You can't think if you're dead. Or Stunned.
Paralysis don't block purely mental actions, so it don't block "thinking".
Slow makes your body... well, slower, but it doesn't affect your thinking.
Web is... well, a web. It hinders your body, not your mind.
Stun, Daze, Dazzle and similar stuff, on the other hand, makes your senses go wacko, so they aren't really blocking your movement. Stun never stopped you from moving, it just stopped you from thinking for a while - and since you don't think, you don't act.
So, the point is,
If something affect your senses, Freedom of Movement can't help you.
Think like a "Houdini Effect". Houdini can escape from almost anything, considering that he
knows what's going on. If you throw him with a concussion inside a closed coffin... well... he will stay there.
So, to determine what Freedom of Movement removes or not, use a simple rule:
Why I can't move?
If you can't move because a spell or something is hindering your body to move, like Web, Freedom of Movement can help you.
If you can't move because a spell or something is making your brain go gonzo, like Stun, or because your body becomes something that can't normally move, like stone from Flesh to Stone*, it won't help you.
*Flesh to Stone don't impede your movement, it merely limits you to the movement that a stone statue is allowed to do. A "Freedom of Movement"-ed and "Web"-ed person would become a completely untangled stone statue.
Also, read Freedom, the 9th level spell:
The subject is freed from spells and effects that restrict its movement, including binding, entangle, grappling, imprisonment, maze, paralysis, petrification, pinning, sleep, slow, stunning, temporal stasis, and web. To free a creature from imprisonment or maze, you must know its name and background, and you must cast this spell at the spot where it was entombed or banished into the maze.
Emphasis mine.
Freedom removes a bunch of effects, like Flesh to Stone and Stun. It would seem rather... strange to say that a way lower-level spell can do almost all the things that a 9th level spell can. Freedom is Freedom, not Freedom of Movement.
Best Answer
There is no hard-and-fast limit.1 Warforged and undead bards can sing indefinitely, seeing as they need neither breathe nor sleep.
For more biologically-limited bards, the limit is whatever makes sense to the table. In this, bardic music is just like all kinds of other not-specifically-strenuous activity: the rules just don’t cover it. Arcane spellcasters need sleep, but there are no rules for staying up for other classes. Running or marching eventually tire you out and force you to stop, but a simple leisurely walk has no rules for when you cannot do that anymore. And yet we know that all humanoids require sleep (or trance) and cannot do such activities indefinitely. So the table has to draw the line somewhere. Bardic music is the same.