Nope, you probably installed a theme with that BGM and then installed on it a wrong-made theme.
GBAtemp.net - The Independent Video Game Community
Rinnegatamante
Sono
That's what everyone says. I have checked both the theme file, and both the bgm file, but neither of those contain this particular music. fyi, I dumped the sounds from the theme with YATAplus, and I played the bgm in EveryFileExplorer.
Sono
Yes, I tested this on my 9.2 SysNAND, and it happens there too! I don't think this is a hidden feature in CHMM 1.5 (.cia), or is it? 
Rinnegatamante
Only one easter egg which apparently nobody never found and that was deleted with v.2.5 ever appeared in CHMM2.
Sono
So it was really an easter egg in CHMM 1.5? Do you still remember how/why it's triggered?
Do you still remember how/why it's triggered?