Since the FM "audio" arrives as "FM MPX" over the tuner port, you'll probably find it much simpler to use the AUX audio inputs instead, which are for conventional analog L/R stereo sound.

Could still use the tuner serial port for control, though. If you want to hijack the existing tuner software interface, _and_ use AUX-in for audio at the same time, then a minor kernel hack will be needed -- to change which "mixer" input gets selected when in "radio" mode.

Cheers