So I was a little disappointed upon starting up Little Big Adventure to find that all the music is in awful midi format!

My Playstation version had beautiful CD audio, I had bought this on GOG because my disc no longer works properly. Is there a way to change this?

There was talk of this issue originally, perhaps this thread will give you the answers you need:

http://www.gog.com/forum/little_big_adventure_series/music_has_aweful_compression_the_origional_was_pure_cd_audio/page1