[Quoth]

mp3 is not purely software. It's a compression system and can be implemented without any software.
But the like patent on FM Radio, perhaps it's mostly mathematics. Patents should never have been issued for the Superhetrodyne or Frequency Modulation systems, because ultimately they are mathematics. Particular implementations would have been copyright.

I have some experience of electronic patents (I had one that I thought invalid). However I don't know enough about mp3 to decide if it really should have had a patent, but it's not a software patent.

The majority of patents are bullying by rich corporations and on the basis of prior art, being simply a recipe, obvious to someone versed in the art, or lacking novelty, or not currently implementable, should never have been granted. The worst in the world is the USPTO because they get more income by approval than rejection and their theory is that the validity should be decided in a USA court, not having no prior art, not being simply a recipe, not obvious to someone versed in the art, ignoring novelty, and ability to implement.
Edison was the most famous start of the absolute abuse of the US Patent system.