Mod Version:
6.3.6
The version that is installed in your pack.

Forge Version:
1.12.2 (forge: 14.23.5.2855)
The actual Forge version - "latest" does not count.

Link to client log:
https://pastebin.com/uwZyADNN

Link to crash log:

Minecraft didn't seem to generate a crash log.

Description:
I tried adding a custom .ogg file to replace the battle music. The default audio provided works perfectly fine, but when i try replacing them with my own with the exact same file name, the game freezes (doesnt crash back to the launcher) whenever a hostile mob notices me.

Problem has been replicated with the mod by itself and its dependencies.

I have searched for similar problems on the closed and open issues tab, and found nothing useful.

Here is the pack I have modified:
BattleMusic.zip

Would honestly be nice if there were public packs like this one, or a generator program that could just generate & compile one for you.

Developer seems inactive/dead anyway, doubt this'll be fixed anytime soon.

I know this is a three-year old issue, but I'm on 1.12.2 and have replaced all four ogg files with my own files and it works without issue. You're just not using the right type of audio files. Sure, those have the .ogg extension but ogg is just a container - it's what is inside that matters. The files you are trying to use appear to be Opus files instead of the usual Vorbis. if you convert them from Opus to Ogg Vorbis the files do seem to work.

EDIT: And your logs say as much: File does not contain Vorbis header in method 'readHeader'. Because they are not Vorbis files.