scumm VM mp3 compression help

Sorry if the answer to this was already posted somewhere, but I recently took my copy of Day of the Tentacle from my ScummVM Dreamcast emulator and tried to port it to my psp. In order to make the file smaller I compressed the monster.sou file into monster.sou.mp3 using the mp3 compression tool available on scummVM's homepage and LAME. The problem is when I start up the game I always get this message
ERROR: startTalkSound: SFX file is not open
I've tried adjusting the settings, but to no avail.

Does anyone here know how to fix this?