http://www.mediafire.com/file/23o1vn1crs95725/Namp64.zip
Thought one of these floppies had the original documentation with it; I'll have to find the other stack.
Here's an extended ROM and batch script to patch it with files. They just get copied to 0x200000, and it plays each track you append in sequence after the last. You'll probably have to execute the batch from the command line or feed it a literal address for Namp.z64, lest it search for it in the directory you drag music from. It should work though, since this script is the one I use to attach music before sending it to the N64.
Just to let you know it can't handle all kinds of mp3s. Anything mpeg 1.0 will probably be fine but not every mp3 will work, especially some with the more esoteric special fields or encodings. It was based off amp 0.7.6, likely with similiar compatibility guidelines.
After a quick test, samples of extracted N64 mp3s (such as Perfect Dark's) work fine.
I'll try to find that documentation. Must have it someplace.
-Zoinkity