M to mute Space to set volume to 100% L to increase volume Q to decrease volume
https://ia802907.us.archive.org/29/items/undertaleost_202004/Undertale%20-%20Lossless%20Soundtrack%20%28toby%20fox%29/toby%20fox%20-%20UNDERTALE%20Soundtrack%20-%2060%20Wrong%20Enemy%20%21-.mp3
