Not about the airplay and handoff, but he did a new username and that was the solution.
Quoting his problem:
Going to Settings->Audio I was surprised to see the Mac Mini under “this Mac”, instead of the M1 MacBook. A safety check intsalling Roon on my wife’s MacBook (she got my former 2019 Intel one) shows the MacBook under “my Mac”.
hope it helps
Traian