The first thing I would try is to remove the virtual sound card from the VM and add it back in again.
I would do that with the VM shut down. It might work when running, but unsure about that.
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Hi gerdgoebel ,
Could you try the following steps?
1.Shutdown the imported W8.1 VM
2.Remove the sound card via VM settings -> Sound Card -> Remove Sound Card,
3.Re-add it via VM settings -> Add Devices... -> Sound Card
Hope that could help.
This resolved my problem. Sound is working in W8.1 and W10 now.