i had the same problem with C-Media usb dac's. so i switched to StarTech and have had no problems since. relevent portion of dmesg follows:
uaudio0 at uhub5 port 2 configuration 1 interface 1 "Speed Dragon USB Advanced Audio Device" rev 2.00/1.04 addr 9 uaudio0: class v1, full-speed, sync, channels: 2 play, 2 rec, 9 ctls audio0 at uaudio0 uhidev3 at uhub5 port 2 configuration 1 interface 3 "Speed Dragon USB Advanced Audio Device" rev 2.00/1.04 addr 9 uhidev3: iclass 3/0, 1 report id uhid3 at uhidev3 reportid 1: input=15, output=15, feature=0
