No sound in guests after upgrade to Bullseye and VB6.1.34

[Raven]

I was using Debian Buster with VBox 6.1.32, and all was well. Now I have upgraded to Bullseye and installed the Debian 11 package of VBox 6.1.34. Now I no longer get sound from any guest. The error is

Host audio backend (PulseAudio) initialization has failed. Selecting the NULL audio backend with the consequence that no sound is audible.
Error ID: HostAudioNotResponding

Bullseye has been out for some time, so maybe a solution is already known. How can I restore sound to my guests?

[scottgus1]

I regret that sound in a VM isn't one of my strong diagnosis strengths. But have you fully shut down the VMs before the upgrade?

And are you running official Virtualbox from https://www.virtualbox.org/wiki/Linux_Downloads, or are you running the Debian fork?

[Raven]

I am using the downloads from this site. I always close all guests and VirtualBox itself before upgrading.