Check to see what your audio device is in Tools > Options > Audio Device . If it's not set explicitly to your headphone audio output, try doing that. That probably won't be it, but it's worth trying. Stop and restart MC after you make any change to the audio device.
There's some possibility that sample rate has something to do with this; but again unlikely. You can look at your sample rate conversion settings in DSP Studio . Again, this probably isn't the problem.
Re-reading your post, it seems this problem just started. Can you think of anything else that has changed on your Mac around the time this problem happened?
I'm tempted to recommend a reinstall of MC30, but you say it happens on 28, 29 *and* 30.
As you can see, I don't have a lot of good ideas here! Good luck.
Brian.