From the User Manual,
Ch. 5.9 CD/DVD support (emphasis/coloring, mine):
As an experimental feature, for additional capabilities, it is possible to give the guest direct access to the CD/DVD host drive by enabling “passthrough” mode. Depending on the host hardware, this may enable three things to work, potentially:
• CD/DVD writing from within the guest, if the host DVD drive is a CD/DVD writer;
• playing audio CDs;
• playing encrypted DVDs.
There is a “Passthrough” checkbox in the GUI dialog for configuring the media attached to a storage controller, or you can use the --passthrough option with VBoxManage storageattach; see chapter 8.18, VBoxManage storageattach, page 145 for details.
Even if pass-through is enabled, unsafe commands, such as updating the drive firmware, will be blocked. Video CD formats are never supported, not even in passthrough mode, and cannot be played from a virtual machine.
To make it simple, if it works, it works. If it doesn't, there's not that much you can do about it, unless you're familiar with low level programming and you're willing to contribute the code the the VirtualBox tree.