If you really want to cut down on the size, then you could just remove MONSTER.SOU. But then you'll get no speech, which is boring :). I recommend Drigo's method, you'll find the compressor tool at:
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/scummvm/tools/)
It's the tool called extract.c. You have to compile it though. It will converts the MONSTER.SOU to a much smaller MONSTER.so3 file. For the extractor to work, you'll need to install the LAME library available at:
http://www.mp3dev.org/mp3/)
You also have to build ScummVM with MAD support, thus needing a MAD library install available at:
http://www.mars.org/home/rob/proj/mpeg/)