Hello,
Lame cant read the .sou files direct since its a collection of many wav-files. You can use the compression-tools provided by the ScummVM Team for this. To get the latest Tools download the latest CVS Builds for Windows: http://scummvm.sourceforge.net/downloads.php (at the bottom of the page).
Use compress_scumm_sou.exe (the successor of sou2mp3) in order to compress the Sou file. Additionally you need a command line compression tool for the Codec you prefer (oggenc for Ogg Vorbis, lame for MP3; copy it to the same dir with monster.sou and compress_scumm_sou.exe). Call compress_scumm_sou.exe without parameters in order to see the options. Also a short description is given in the Readme File in the Tools Dir. For compressing the .sou file to MP3 with standard options call
compress_scumm_sou.exe monster.sou
or
compress_scumm_sou.exe --vorbis monster.sou
for Ogg support. The Result will bw a monster.so3 (or monster.sog). You can delete monster.sou at this point. It will take several minutes in order to compress the file (>10 Min on Athlon XP 2000)!
Kind Regards
Frank