Songs do affect the singing bard, battle voice also affects the singing bard. Your MP will almost not dwindle with Ballad paired with Battle Voice. Battle Voice should be used only for 2 scenarios : Recover 25% of Healers MP, expecting a burn phase when you have a Black Mage in group with Requiem.

In any other situation outside where there the above 2 situation doesn't occur. Don't use Battle Voice, and only requiem till 50% of your MP and toggle it off and on when you're almost full again.