This Google doc tutorial (w/pictures) explains how to modify Sonic Mania's music/SFX files for the purpose of speedrunning.

The link for it can be found here: https://docs.google.com/document/d/1VLVlHX1EH0tSrJACvkUPkxcQv-cyPcuiF9EokXnyzrE/edit?usp=sharing

If you have any questions or concerns regarding this tutorial then feel free to DM me.