/etc/vdr/plugins/mplayersources.conf is in vdr-plugin-mplayer 0.10.2-22.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | /var/lib/vdr-plugin-mplayer/DVD-VCD;DVD oder VCD;0
/media/cdrom;CDROM;1
|