It seems like I've seen Music Choice more than one place, so it may be different on different feeds, however I just tuned in the MC on G14, and looked at it with TSREADER. Below is what TSREADER says for one of the channels:
Elementary Stream PID 467 (0x01d3) AC-3 Audio
AC3: Bitrate 192 Kbps Sample Rate 48 KHz
AC3: Mode complete main Coding 2/0 L, R
AC3: Dolby Surround Mode not indicated
AC3: LFE Mode Off Dialogue normalization -31 dB
I checked about a dozen of the channels, and every one came up like the above.
Also, I looked at the actual bitrate of the PIDs of each audio channel, and they were all ~ 201.91 kbps
Hope this is what you're looking for. I didn't see any indication of variable bitrate. All seemed to be constant and the same. However maybe there is another feed that does the variable stuff. I think DTV used to have MC audio, and I think DTV may use variable bitrates, however I think they changed to XM a couple years ago.
EDIT:
Another fun thing I can do with the new Genpix that I couldn't do a week ago. Nice to be able to pull up these DCII channels on my laptop.