This cart is basically a demonstration of a simple tool/function I've made to check/debug the music playing in one of my project, the function need no external dependency and should not clobber your variables as all of them are declared as local. You only need to call the function with the Y position as parameter (positive number are relative to the top of the screen, negative from the bottom) in your update screen function to display a bar with the current status of music playing.
There are four zone, one for each channel, the green/red bubble indicate is the channel is playing or not (green == playing, red, not playing) the number next to the bubble is the pattern used on that channel, and the bar on the bottom is the current position in the pattern.
The function itself:
18/05/2016: Version 0.2 - New functionalities, see below
Version 0.2r2: correct a bug on effect calculation
I hope it will be useful to someone :)
I used the demo Woo as a base to demonstrate the function in the demo cart.