A 486 DX4-100 could play mp3s in stereo (or in mono at 66mhz), but do absolutely nothing else at the same time. I used a DOS mp3 player (mpxplay) and it could be done.
Docs suggest stereo is possible at DX2-80mhz if you disabled screen output and heavy mp3 file pre-buffering.
Top level comment here claims the issue was the on-screen animations and they were able to build a highly optimized mp3 player on a 286 (dunno through what speaker): https://m.youtube.com/watch?v=b0zZpzxHSeM
Even on a later pentium, I had to minimize throttle priority on my web browser because smooth scrolling requires a ton of juice. Still does to this day looking at power consumption on an iPhone.