What music exactly? To my knowledge Mutant Mod doesn't add any music.

Is it one of these? https://terraria.wiki.gg/wiki/Category:Terraria:_Otherworld_Music

Yes, it is! Why would Fargo's cause that? I removed EVERYTHING else and made a new character/seed. I've been using this mod for years, I've never heard this music before

Did you ever play on the seeds "05162020" or "5162020"? That's how you unlock the Otherworld soundtrack in vanilla permanently, across any number of future characters/worlds. Once you unlock it, a Music option appears in Party Girl's menu. I'm not sure what happens if you toggle it with her then make a new world.

I read that in Drunk Worlds, otherworld's soundtrack will replace the usual soundtrack each time you load the world. Make sure you don't have "Drunk seed: Always" in Mutant Mod config.

Anyway, if it's truly caused by mutant mod, it might be a bug. You can try joining the fargo mod discord and posting about it there. It's very active. Make sure to include screenshots of your config, maybe a video clip. Check your tmodloader logs to see if there's anything relevant.

So this mod caused every seed to default to the alt OST. Easy fix, just talk to the party girl. Still no clue why the change was force upon seed generation, but it's no biggie. I feel really dumb now, I've been playing this game for years and I'd never heard the alt OST.

I'm not sure why that was happening either. I blame Terraria's / TModLoader's janky sound engine. at least 50% of the bizarre issues I've ever had happening in TML are sound related ones, and there are still a few that plague me to this day, like random super big audio stutters or ear hurting pops. Sometimes when modded or resource pack music starts, I can hear the original track trying to fade in before quickly being cut off by the replacement.

There are also issues with some modded flags being carried over to other players/worlds. For instance with Boss Checklist starting a new world and already saying I've defeated some HM calamity bosses, despite a fresh char/world.

I've had similar progress retention BS happening in other mods and given that no one else seems to have this issue with Mutant Mod, I can only imagine this is some absurd amalgamation of multiple of these things, resulting the phenomena you described.

Like I said you can try joining fargocord and providing logs or other info. If it really is a bug with mutant mod, they'd surely like to know about it. Maybe someone else there is experiencing it and will see your post.

@Kanaris Do you have more information or a link about the sound engine issues?

When I played the game, loud sound effects (like block breaking sound) would sometimes play even if I completely muted sound with the in-game sliders.

I think that this bug combined with lag (old laptop, can heat up a lot during play) caused the built-in (and later, external, both left ear, nothing bad happened to the right one) speaker to start buzzing at some frequencies (even outside of the game).
Nothing like this happened with other games.