The project rate is 44100 Hz. I've tried lowering playback volume of the backing track to almost nothing, and disabled the meter for the mic. I've also changed the refresh rate on the playback to only once per second. Changing the mic recording volume doesn't change anything. I'm using a ProLogic headset/mic through the USB, and am recording to one (mono) channel.
This problem just started recently, and I don't know why, since I hadn't changed any settings. I've been able to record up to 20 tracks with no problem, but now, I can't even put one down.
CPU usage shows pretty low when I'm recording, too.
The problem isn't in the mic, as other programs record my voice just fine, including while overdubbing.
I'm running Mac OS 10.9.5, and Audacity 2.1.2