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Use the In-Game Music Slider

This mod plays its soundtrack through the normal music channel, so the simplest way to silence it is the vanilla sound options. Open Options > Music & Sound and drag the Music slider down. This lowers or mutes the raid track along with other background music. If you still want ambient music but not the raid theme, this will not separate them, so consider the alternatives below.

Disable the Mod or Use Another Music Mod

If the raid track specifically bothers you:

Remove or disable the mod from your mods folder; raids will still work exactly as in vanilla, just without the custom soundtrack.
If you run a music-overhaul mod, check its config for a raid or event music toggle, since some let you exclude specific events.
On a server, remember the mod must be present client-side for you to hear it; other players without it simply hear vanilla silence during raids.

Lowering the slider is the fastest fix mid-raid, since changes apply immediately without restarting the game.