Running this game while having to listen to these particular SFX (BOING BOING) can be grating, so here's a simple file swap to mute those particular SFX. Thankfully Gurumin's audio files are mostly grouped together in a logical order, so these were easy to find (they're all in the gse section). If you want to mute any other SFX or whatever, I used itmcnv.exe to extract the WAV files, muted them in Audacity, then used itmcnv to convert them back into .itm files. I tested it, seems to work fine but let me know if I forgot an SFX or whatever.

https://www.mediafire.com/folder/d5vcr3v5j5i4l/Gurumin