My friend's unit (8gb) is playing songs at about 50% speed and when movies are played the audio track is also slow...can a firmware upgrade do the trick?
as there is no speed-setting option in the M6 this is probably either a n odd firmware thing(update firmware like iversonjack says) or your files are converted badly
May just be an issue with the sample frequency of the audio files in question - 44100Hz - 48000Hz - 32000Hz. Check out the ones that don't play correctly.