I'm confused, my 20 year old 1GHz AMD T-Bird handled audio just fine. Why is this still an issue that needs AVX-512 in 2021?

I don't know about AVX-512 but usually AVX and SSE is used in audio, as one thing is audio playback (where you just copy values to the audio buffer and deliver it, low CPU usage) and the other is audio processing (doing FFTs, synthesizing audio, etc). There you need a lot of single thread processing power as you may be doing lots of calculus in a relatively small vector (64, 128, 256...).

It doesn't make sense to use CUDA for, let's say, a 64 size vector as communication costs more than what you win. I.e a 32 buffer size at 48Khz alredy goes in the microseconds mark of time you have to deliver the audio block, even less if 96Khz or higher samplerates.