Customer may call in to report the error in VM Ware.
Backups fail due to an unsupported audio card
Have the customer remove the HD card from the VM, they will most likely see an error that the card is unsupported. This has also occurred when a customer takes a physical machine and converts it to a VM.
Once it is removed have the customer run a backup, if it is going to fail for this error it would do it within seconds.
For more detailed information on backup failures and performance issues see Unitrends KB 5062 - Backup Failures and Performance Issues