Please find attached a patch adding support for MP3 decoding using
libmad. This is against release ffmpeg-5.0.1, and works for libmad
0.15.1b (the most recent available).

I hope that this is useful. Some context - I found the FFmpeg inbuild
fixed point MP3 decoder produced very distorted audio for low bitrate
streams (56k/s and lower). This was on an ARMv4 CPU for which I needed
to make some adaptation to the fixed point maths routines which
currently target more recent CPUs. I was not able to resolve the
distortion issue and it was easier to integrate libmad decoding which is
already optimised for very fast decoding on the ARMv4 CPU. More info
about this application here:
http://www.megapico.co.uk/sharpfin/mediaserver.html.

Best regards, David.