problem solved by downgrading virtual hardware version to version 8 :-)
update: one must actually downgrade to the virtual hardware version 7.
So what works is:
- Shut down the VM
- downgrade virtual hardware version to 7
- add sound card (if you added it previously, it got automatically removed on downgrade to version 7)
- immediately (without powering up) upgrade virtual hardware version back to version 9
- add networking adapter (it got remove on downgrade to version 7)
- power up the machine
- install the sound driver WoodyZ linked (including needed restart)
Then sound should work.