Solved: Increase playback buffer. Running multitrack was too much for the Mac.
(Addendum: This problem only occurs with Audacity, not when playing sound/music via any other means.)
I'm getting distortion at unpredictable times (but now pretty much within seconds) on playback through both USB ports > DAC on my late-2011 MacBook Pro:
Mac OS X Lion 10.7.5
Intel i7 at 2.8 GHz
4 GB 1333 MHz DDR3
Sometimes it distorts and fades to silence, and at other times it just distorts. If I stop and restart it may play through or may distort quickly. This does not happen on the built-in 1/8" output. I have a new Audioengine D1 DAC. Used it earlier in the week through a pair of Audioengine 2 with Dayton 8" sub.. Sounded nice. During that first listening session, I started hearing distortion on playback only from my four-track Audacity .aup files (24/44.1). I am saving and running files from a WD 3TB drive.
Fix attempts so far:
Exchanged the DAC with another new one.
Updated Audacity from 2.1 to 2.1.1.
Reset PRAM.
Tried both USB ports.
I did use a crappy NuForce DAC in the past with no problems.
Anyone ever heard of this problem?