PS I think there is a bug in the new build. I can no longer choose which output to use while JRiver is playing. The Mac remains set on whatever output was being used before JRiver starts playing a son. Not having the problem with any other players. I'm done with this for awhile. Thanks everyone.
Switching outputs, using the Mac's Sound switcher in the top menu bar, works with apps that play to Mac Core Audio. This really isn't how MC was designed to be used. MC is designed to play to a specific named audio device. You choose this device in Tools > Options > Audio > Audio Device .
Presumably, you now have a specific device selected in that MC options area. If want to go back to choosing outputs, on the fly, using the Mac's sound device switcher, then you should change the MC audio device to [Default] (Core Audio).
There's some possibility that this will change the way it sounds. Since all of the other players that you like the sound of are using Core Audio, maybe this will be a positive change. It's worth a try.
As with all "major changes" to MC, you'll want to stop and then restart MC once you've changed the setting.
Brian.