Top Mall-Fi poster. The T in META42. Formerly with Tangentsoft Parts Store
Sep 27, 2001
When the voltage gets low enough, the amp starts "clipping", which is easy to hear even in tiny amounts. Once the amp starts clipping, you have to change out the battery because the music becomes unlistenable. Since batteries drop in voltage over time, the the higher voltage you start from, the longer it will run before you have to change the battery.
Also, op-amps sound better at higher voltages. This effect is especially true at single-digit voltages. (Beyond about 12V, most chips don't improve much.)
With most types of rechargeable batteries, you usually get a quick voltage drop-off when the battery is freshly charged, then it stays more or less at the same voltage for a long time, and at the end of its life it drops quickly again. Presumably a 9.6V rechargeable will have a higher-voltage flat area than a 7.2V or 8.4V rechargeable battery.