Yes, if the player wants it to be music, then it's music. It specifically gives the Bard character that option. I don't know what Requiem does, but if it can somehow take advantage of this while the bard is casting a spell, then it sounds legit.
I believe it originally appeared in Song & Silence, but was updated for 3.5 in Libris Mortis. I believe it allows mind-affecting abilities to affect the undead. I don't have the specific language in front of me, but IIRC it's specifically designed for bard songs.
No. Your bard spells are cast using music. They are NOT uses of the Bardic Music class feature.
Thus, Requiem will not affect your spellcasting in any way. It only enhances the effects of your Bardic Music class feature.