Actusreus,
Jea48 has more knowledge on this subject than I do. So, if
I say or recommend anything that differs from him, please follow his advice. That said....the bridged breakers are for 240 volt branch circuits. The twin (not bridged) breakers are for 2 independant 120 volt branch circuits.
The wall outlet that you want to plug the stereo into, will be fed by one of the 20 amp (non bridged) breakers.
With a 20 circuit you shouldn't have any problems running your stereo. To make sure, as I mentioned earlier, turn off the breaker that powers that particular outlet and check to see what else is on the same circuit. While doing so remember that each 20 amp breaker lever controls one circuit. Don't turn 2 off at at time, it will skew your results.