After some hacking on this on the airplane back from India, and some finishing touches today, the audio output on the M800 is now working under Linux. The AK4641 codec driver and M800 ASoC device driver are to be found in the gnufiish git tree.
What I yet have to verify are the interconnections with the GSM Modem audio, as well as the FM radio. At least general audio playback is working, both through the earphone and ringtone speakers, as well as on the headset.