So, I saw a suggestion on r/Minecraft talking about biome-specific music, and wondered if it was possible with resource packs? I doubt it is, but i know there are some Minecraft gods out there. I also have Optifine, if that's any help.
Because of the way Minecraft resource packs work, no, it is not possible to create biome-specific music with resource packs alone. Minecraft activates the default music in certain situations (survival, creative, nether, credits), and the only thing a resource pack can do is change what music plays in these situations, not add new situations of music. You can, however, add custom sounds and music with resource packs that can be played with the /playsound command as well as other datapacks, which can run the command if a certain condition is true, like if the player is in a specific biome. Trigger the Music, a datapack & resource pack combination that I made a while ago, for example, does just this.