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Custom music in level C5

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I might still be living in 2010 here, but editing the value at x1924 in a hex editor to match the value of the track you want to use should do the trick.
Or you could use this, which makes it easier if you ever change your mind on which music track to use there:

Originally posted by the ROM map
Changing $009725 (0x01925) to "9C FB 1D" will allow you to change the music for the intro via Lunar Magic.

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Get the official ASMT resource pack here!

I found out inserting a new intro song through AMK seems to do that rom map hex edit ycz was talking about. It saved me some trouble.
Polaris is cool. I think. Can't confirm.