I not renember well but think gamestudio support that.
but not all the windows versions? support adpcm compresion
in wav files (can be 4 bit audio, only for voice) and then the files are more small

"ADPCM Audio Codec" coming with all version of windows
probably you can save with audio software in that format
diferent types of compression if you like music but with very bad quality
but for voice are perfect, think the speex if for use if your program in other operating system, but gamestudio only runs in windows