..however, having a few conversations with more knowledgable people has changed our mind and we're definitely going to give it a try on Metal and Vulkan to see what happens. Should be interesting.
I don't have any experience with Metal or Vulcan but my intuition is that audio DSP is going to include a healthy dose of linear algebra and multi-variate calculus. That points towards some kind of fit with the GPU. Given that basic GPUs are available on practically every device (including phones) it seems like a fantastic fit. Even audio hardware developers would benefit since it would open up access to commodity priced chips (rather than custom asic/fpga/whatever).
And even for very simple audio loads, there's often unused capacity in compute cores (even when running games) which would could do the job "for free" without bothering the CPU.
Also, we do know a few people who want to use SOUL to write audio code which does need high parallelism. It's not a super-common use-case for audio, but it does exist.