I was used VirtualBox 5.2.18 on Windows 7 host and Windows 10 Version 19H1 guests via support by Microsoft RD Client for Mac, until last weekend.
The remote sound output of VRDP was working so good, the sound mirrored perfectly by RD Client for Mac.
I updated VirtualBox from 5.2.18 to 5.2.22, in yesterday. it seems that no problem in VirtualBox guests environment(s) and for operations.
But only the sound mirroring on VRDP is troubled. No sound hearing from Internal speaker on Mac even through Microsoft RD Client for Mac can communicate to
VirtualBox guest machine on remote host.
I'm trying to downgrade VirtualBox to 5.2.18, and have restored sound mirroring on VRDP, now. Please inform me to solve this problem in new version.