L to increase volume Q to decrease volume M to mute Space to set volume to 100%
https://ia902907.us.archive.org/29/items/undertaleost_202004/Undertale%20-%20Lossless%20Soundtrack%20%28toby%20fox%29/toby%20fox%20-%20UNDERTALE%20Soundtrack%20-%2079%20Your%20Best%20Nightmare.mp3
