Being able to actually cast an unlimited number of spells is--barring excessive shenanigans via things like using plane shift to go to planes wherein time passes at a far faster rate so you can rest in the middle of an encounter--impossible. Having a character with a host of at-will spell-like abilities is possible with some effort.
Templates
If the character starts as a humanoid, he can take the templates half-fey (Fiend Folio 96) with at-will charm person for LA +2, half-rakshasa (Dragon #313 96) with at-will empathy (Dragon #313 94) for LA +3, and katane (Dragon #313 64) with at-will spider climb for LA +4, then top off with either half-celestial (MM 144) with at-will daylight for LA +4 or saint (BE 184) with at will bless, guidance, resistance, and virtue for LA +2. The character'll have 1 HD when he takes his 1st class level and count as a level 12 or level 14 character, but his ability scores will be amazing.
Also, this assumes reflavoring the names so the character isn't 3/2+ something.
Races
Play a monster. All of the following monsters get at least a few spell-like abilities at will: the ECL 20 astra deva (MM 11) and trumpet archon (MM 17); the ECL 19 hezrou (MM 44); the ECL 18 vrock (MM 48); the ECL 16 couatl (MM 37) and erinyes (MM 54); the ECL 11 hound archon (MM 16); the ECL 12 succubus (MM 47); the ECL 10 drider (MM 89). You can probably tell where I stopped going through the Monster Manual, but picking a monster is almost always suboptimal to picking an actual non-LA, non-ECL character.
Classes
The warlock from Complete Arcane's been suggested, but I also recommend Tome of Magic's binder, whose vestiges can grant spell-like abilities at will, like the Acererak vestige which grants at-will use of detect undead and speak with dead. The bound vestige persists for 24 hours after which it must be bound again, so it's not technically inherent-forever spell-like abilities. (Thus foiling your scheme to create a perpetual motion machine via an enslaved creature with an at-will spell-like ability.)
A 5th-level spellthief can steal another's at-will spell-like abilities all day long. The spellthief only gets one use of it before he must steal it again, but if the source is limitless so is the spellthief's ability to steal. Improved familiars make good targets.
Feats
Other than Complete Arcane's feat Innate Spell--which lets you trade mind blank or polymorph any object once per day for mage hand at will--getting at-will spell-like abilities via feats looks impossible.
However, like everything in D&D 3.X, being a caster and ruthlessly pushing the limits of the feat Craft Wondrous Item (PH 92-3) pretty much lets you make anything your DM allows. With the core rules setting a precedent with hand of the mage (DMG 258), convincing your DM to allow crafting other comparable at-will items should be trivial.
There's one feat, though, that I mention here because it's the Feats section: Fiendish Summoning Specialist (PlH 39). You needn't take it; just know it exists.
Spells
Depending on the errata the campaign uses the spell polymorph any object might be used to just become a creature with spell-like abilities, but neither magic jar nor shapechange grants access to a creature's spell-like abilities.
What does explicitly grant access to a creature's spell-like abilities is the 5th-level Sor/Wiz spell fiendform (SpC 90). It reads, "[Y]ou can take the form of any fiendish creature, demon, or devil that can be summoned by a summon monster I, II, III, IV spell..." (emphasis mine). So if the Fiendish Summoning Specialist feat exists in the campaign, by a strict reading, other folks have added to the summon monster lists, and those can be accessed by anyone who casts fiendform. The fiendform spell only lasts 1 minute per caster level and has as an unpriced but weird material component, and you've to convince your DM that other people totally put the demon, devil, or fiendish creature you want to change into on their summon monster list via the Fiendish Summoning Specialist feat, but if those obstacles are overcome--and you don't mind that sick feeling you get after having so much cheese--, have fun with your demonic and devilish at-will spell-like abilities. If being such a creature all day is required the fiendform spell is a valid target for the Persistent Spell feat (CAr 81) and multiple ways exist to reduce what would otherwise be an 11th-level spell to something that can be cast pre-epic.
There is not, to my knowledge, an identical opposite celestialform spell.
Best Answer
I'd go for a cantrip. Thunderclap (from the free Elemental Evil Player's Companion) is a good bet; it's short-range but it has the auditory theme. Its somatic component could be you stopping and howling loud into the air.
If your GM will let your Sorcerer take a Bard cantrip, consider Vicious Mockery. It deals psychic damage, but you could interpret that as your howl being unnaturally unsettling.
Reskinning: If the above don't work for you, you can also reskin an existing spell that does a different damage type to deal thunder damage instead. Be aware that not all damage types are created equal; for example, more monsters are resistant against fire and cold than thunder, so converting one of those spells to thunder will actually increase its power.
My suggestion if you want to reskin would be to find a force- or radiant-dealing cantrip and turn it into a thunder-dealing one. You'd be decreasing that cantrip's power a bit but not enough to really matter.