Same Auto-Com Pro AVI unit here as well. No problems with XM, Mp3, or Navigation unit. I'm using in-ear phones. You?
You didn't say whether you set the Individual Devices volume control (what was at 100%?), or the Master Volume control either.
As to the car unit, I only used it a few times and it was fine although it was turned up maybe more than when used on the bike. However, the quality of the Zumo car-mount speaker is pretty poor. Depends on how noisy your car is also. It may be better on the bike.