Ken Silverman, creator of the BUILD Engine, made MP3PC.exe that's still hosted on his Utilities page. I'm pretty sure I used this on a 486 years ago and was surprised at how well it worked, though it apparently is picky about MP3 encodings. From the description:

> MP3PC.EXE (73,483 bytes): Got no sound card? MP3PC plays MP3 files through your PC speaker! For some reason I like to make programs work through the PC speaker. Some people still don't know that it's possible to play real sounds through it. Unfortunately, you must run this in pure DOS mode for it to work.

His Utilities page can be found at: http://advsys.net/ken/utils.htm