In case you have assigned a physical CD or DVD drive to a guest and the guest does not notice when the medium changes, make sure that the Windows media change notification (MCN) feature is not turned off. This is represented by the following key in the Windows registry:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Cdrom\Autorun
Certain applications may disable this key against Microsoft's
advice. If it is set to 0, change it to 1 and reboot your
system.