pxtone comes with its own DLL for plugging into games to allow them to play pxtone tune files. However, it is incompatible with Game Maker in its natural state. pxtoneGM fixes this problem by providing an interface for Game Maker to use pxtone with little hassle as possible.
Click here to download pxtoneGM v188.8.131.52
I'm aware that while moving the pxtoneGM player window the music fades out. This is because I'm using a work around to get the current playing sample through the pxtone_fadeout function. There is no function in pxtone to just obtain the current sample. I believe it was implemented this way to just allow pausing and resuming the music (it is for games after all, not a music player). I'm not really concerned about the player anyway. It's just an example so that you can hear some wonderful pxtone tunes in Game Maker.
Where do I get pxtone tune files?
By making them with pxtone. pxtone has only been around a few years and has yet to be finished so it doesn't have a very big userbase yet -- especially not an English-speaking one since the program is by a Japanese developer. It is almost impossible to find any pxtone tunes by themselves on the net. That said, Pixel's site links to various sites that share pxtone tune files publicly.
pxtone can be downloaded by the music note icon on Pixel's website next to 2008.
pxtoneGM is released under a GPL license.
I believe Pixel asserts that there is no limitation on pxtone's use but asks politely to attribute him for his work. To do so he asks for you to use the banner he provides, pxtone.bmp, in anyway you like. Don't take my word for it though because my Japanese isn't very good. His original statement in Japanese can be found in the pxtone_include folder of pxtone in ReadMe.txt.