I know moonshell, but it sttuters sometimes in my setup, and also another option would be welcome. I believe there's a GBA movie player already, right?
Fix your codecs, decrease some bitrates and you shouldnt get stutter.
There is a GBA movie player ROM maker called meteos, but it is old and REALLY shows it. There is a GBA MP3 player too, same deal and its not integrated into the same program.
This I think is to add some additional uses to the Neo2, while not taking the route of M3 by closing it up like they jelously do with their "crystal engine", also giving the community of flash cart users another option for their GBA/micro etc.