Hello again!
This time I'm going to show you a music player... for Game Boy. :P It's very easy to use, and easy to compose music for it. I've included source code (100% asm) and templates for RGBDS and GBDK. You have to compose the music in a tracker in the PC using a custom mod file included in the download. Available effects and other instructions to modify it are in a text file in the download. Then, you convert it using a special tool which will export the data to a GBDK c file or a RGBDS asm file. Included templates can be compiled by double clicking the bat files, so you can use them for testing your songs as you compose them (I'd use the RGBDS one). For any other information, look at the readme, or ask me. There is an example there, the rest should be self-explanatory.
PS: Is there anybody who still codes for GB? :P
Update
Some wrong definitions changed:
Update
Updated some links to point at the correct locations of the tools as of 2018-01-29.