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From Sonic Retro

(Original guide by lukeusher123)

To add more music slots to the sound test, search for this code in the level select routine:

LevSel_NoCheat:
        cmpi.w    #$94,d0    ; is sound $80-$94 being played?
        bcs.s    LevSel_PlaySnd; if yes, branch
        cmpi.w    #$A0,d0    ; is sound $95-$A0 being played?
        bcs.s    LevelSelect; if yes, branch

Change the line:

        cmpi.w    #$94,d0    ; is sound $80-$94 being played?

So it reads:

        cmpi.w    #$9F,d0    ; is sound $80-$9F being played?

Then rebuild the rom.

And now the sound test will have the music slots going up to $9F, instead of $94, however attempts to play empty music slots will crash the sound driver, and possibly lock up the emulator. So, make sure you include a song in every slot.