What's strange is that every day, when I start playing, I have to unmute (in the Windows Volume Mixer) one or more of my 4 instances.
I DO have the "mute background game audio" checked in my character set, but if I don't un-mute the muted instances when I start boxing, then ISboxer isn't able to properly un-mute a muted instance when I switch to it. Not sure if this is an anomaly with SWTOR or a bug in ISBoxer, but since the workaround is easy, I'm not gonna sweat it much.