I don't see a bug tracking this, so I think it's best you create a debug log and include details on your audio setup (card/headset, make/model): http://mediamonkey.com/forum/viewtopic.php?f=7&t=69 and sent it all to support.

It's set to "primary sound driver." I realize I can change this, but it would be nice if MM would stay calm when the "primary sound driver" changes from BT to sound card.

You probably need to change the Audio Device in the enabled output plugin (Tools > Options > Output Plug-ins). Others have reported the same issue.

WMP just reroutes audio back to the speakers when I disconnect by S305 bluetooth headset. Mediamonkey plows through my entire library as if it can't find a single tune and then sits there, in a corner, shaking back and forth, until I shut it down and restart.