To add to tomwhore's comments about the possibilities of hacking the rio volt firmware - I don't know if it's been done yet but I know it would open up the opportunity to do some pretty interesting stuff.

AFAIK support for a new version of WMA was included in a firmware upgrade released a few months after the launch. If, memory-permitting, this could be done with other audio file formats such as Ogg Vorbis, the lifetime of this player could be significantly enhanced (and I'd guess the longevity issue was one of the reasons upgradable firmware was included in the first place).

Unfortunately, I'd say the chances of this free format being included alongside WMA are slim to none. On the other hand, although figuring out the player's flashing specifications could be time-consuming, there's unlikely to be any serious encryption done during the flashing stage... or is there?