MMX is integer math only. MP3s require floating point unless you hand code fixed point version of the decoder. In real life just recompiling with MMX support gives marginal difference https://www.cs.cmu.edu/~barbic/cs-740/mmx_project.html
MMX was pretty useless (reuses FPU registers = cant run FPU code in parallel) marketing gimmick from Intel designed to tick boxes, promoted with fake "designed for MMX" campaign https://www.mobygames.com/images/covers/l/51358-pod-windows-... spoilers: MMX enables one sound filter in the whole game, no speed difference. Ubisoft just made some extra cash by printing this on the box.
MMX was one of Intel's many Native Signal Processing (NSP) initiatives. They had plenty of ideas for making PCs dependent on Intel hardware, something Nvidia is really good at these days (physx, cuda, hairworks, gameworks). Thankfully Microsoft was quick to kill their other fancy plans https://www.theregister.co.uk/1998/11/11/microsoft_said_drop... Microsoft did the same thing to Creative with Vista killing DirectAudio, out of fear that one company was gripping positional audio monopoly on their platform.
> MP3s require floating point unless you hand code fixed point version of the decoder.
This is a weird statement. "MP3 encode/decode requires floating point unless you implement in fixed point such that you don't need floating point." It's perfectly possible to write fixed point MP3 decoders.
Sure, MMX wasn't that great, but it was Intel's first SIMD extension, was definitely intended to help with "things like MP3 decoding," and was followed by a ton of improved extensions with similar goals.
Intel MMX (multimedia extensions) introduced dedicated hardware to accelerate things like MP3 decoding.