On Genos,
This is another method, No Multipads but audio file.
In the midi file you can put a sysex command 'Start Audio' (Can easily be done with MixMaster)
When the midi file is played, this command starts palying the loaded audio file.
The MBT of the Start command must be exactly on the right place.
This is sometime a little bit error and trying.
But once done it works perfect.
The only thing you have to take into account is that the audio file has the same tempo as the midi file.
Myself I have several audio backing tracks from 'karaokeversie.nl'.
You need to have a backing track with the same tempo, not a variable tempo.
I don't know if on the S770 you can load a audio file.
Regards
Etienne