Same issue here with Big Sur Beta 7 on my 2019 MacBook Pro. Interestingly, I still hear that garbled echo sound if I mute my MacBook. Did somebody reported this bug already to Apple?
Short Katz
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Accoring to this comment in LLVM code, Apple M4 is ARMv9.2a without support for SVE.
// Technically apple-m4 is ARMv9.2a, but a quirk of LLVM defines v9.0 as
// requiring SVE, which is optional according to the Arm ARM and not
// supported by the core. ARMv8.7a is the next closest choice.
https://github.com/llvm/llvm-project/commit/2b33591386e855cf82a4104092e1a1d2888f32b1