My USB Audio Interface doesn't play music after upgrading to OS 4.0 on my iPhone/iPod touch.
A hard reset (complete power off) to the device should resolve this problem. To perform a hard reset, hold the home button (below the screen) and the stand-by button (on the top) until the device reboots (ignore the "slide to power off" and wait until the Apple logo appears).

Note: if a non-reset OS 4.0 iPhone/iPod touch is connected to the vehicle and disconnected, other OS 3.x iPods/iPhones may not work until the vehicle has been restarted.
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