Apologies if this has already been answered, but I had the same problem with the Indiana Jones Atlantis monster.sou file. The original file is 146mb, and using sou2mp3 produced a file that was about 47mb - stil too big for me until I get more storage space.
If you download the tools including the file extract.exe from :
http://prdownloads.sourceforge.net/scum ... p?download
and also lame.exe (can't remember where I got it from but I just searched for it) and place them into the same directory as your monster.sou file then you can use extract program to create a smaller monster.so3 file than you can with sou2mp3.
By reducing the quality I was able to produce a monster.so3 file that is 26mb (works OK) and one that is 15mb (haven't tested yet). The 26mb file was created using the following command :
extract -b 16 -B 16 -V 5 -q 5 monster.sou
just type in extract and it will give you a list of the different parameters. Just decrease the bitrate and quality for a smaller file.
Hope it helps.