To play DVDs within Qubes I have used my new multimedia App VM (which can also play Spotify and Netflix, see post from yesterday).
The multimedia App-VM is based on the Debian 8 Template in Qubes 3.2
Some suggested to install the whole desktop within the Debian Image, I don't want to install everything but only the stuff, that is really needed.
As such I have looked at the Debian documentation
1) Launch Debian 8 Template VM and install the Qubes Proxy Tools, so that you can pass your (external?) DVD-Drive via sys-usb to the multimedia App-VM. Strangely the qubes-usb-proxy seems to be installed within the fedora template but not in the debian template.
In Debian Template VM:
sudo apt-get install qubes-usb-proxy
2) Install libdbdread4 from the default repositories:
In Debian Template VM:
apt-get install libdvdread4
3) Install libdvdcss for the decryption of CSS protected-DVDs.
Because of license restrictions in various countries this can't be done from the Debian repositories but needs to be installed manually.
(not that hard :-)).
Download the packaged .deb from: http://www.deb-multimedia.org/dists/stable/main/binary-amd64/package/libdvdcss2