Hey, I was wondering what exactly (and now by game mechanics) are the components to bard's spells, and wether a bard can hide his casting (lest say, while performng, or just playing music)?

My recollection is that the engine doesn't even allow a bard to take silent spell and cast bard spells silently, so I'd say it's safe to assume that a bard spell will need some vocal component and quite likely be typically musical in nature. However, there's plenty of flexibility there to match any particular bard (primitive chanting, hissing and snarling, melodious humming, etc.)

Regardless of the bard, I'd say pretty confidently that if a bard did cast a spell then it would be a recognizable use of magic. Certainly if another character passes a spellcraft check, they'd be aware of what was going on.

What happens if a mage (wizard, sorcerer) sees him and fails the spellcraft check? I assume normally a wizard will identify another wizard casting, even when failing the spellcraft.