I played mp3s on a 486dx2... I think at 40MHz (but could be after 100MHz upgrade). The biggest issue was bitrate, I think it struggled with 128kps but was fine if they were encoded anywhere below. Certainly could not play 160kps. 96kps mp3s were common enough on the internet back then with dialup, although they sounded awful.
Brings back memories of the mp3car (PC parts) and mp3ar (MAS3507d) projects. Going back to 2000 on archive.org https://web.archive.org/web/20000408180856/http://www.mp3ar....