Very useful patch. If you are using retry, make sure to set $7E14AD and $7E14AE to 0 in the retry-extra death routine. Otherwise, the pswitch timer will persist between lives and will most certainly break the level.
Do I need to make the AMK changes if I'm not having a P-Switch song? I want to use this patch and I wonder if not having the P-Switch music at all would also clear this problem with music in different levels.
Also just note you can use different music between rooms with the patch and Addmusick; you just can't use different music between rooms while a p-switch or star is active or it won't work correctly.
You can work around this by clearing the timer before you load a new level (UberASM or something), or by just not giving the player the option of hitting a POW, or getting a star, and being able to go into another room.
Like it says in the readme though, if it is the same song there should be no problem.