You mention a Yuasa Pb-acid; then a Shoral Li-Ion. Wich are you inquiring about?
I know from experience a new Yuasa Pb-Acid, if simply left to go dead, will actually come back after 3y/10mo. But, this is exceptional. I would not expect a Li-Ion or any battery for that matter to be any good if a constant drain on it.
Get an isolator diode so the sound system only draws from alternator or a switch like Lady Di says.
Diode: Find stereo's total WPC rating, multiply times # of channels; divide by 12.6, multiply by 1.5. Use that forward current-rated Shottkey diode rated for 50 PIV. (EBay) Remove the stereo and reg/rect hot wires from battery, attach them together thru suitable fuses to the Unbanded end of diode. Banded end to battery.