Asus Xonar D2X - Manual (Page 15)

9 ASUS Xonar D2
3.4 Connecting CD/DVD-ROM Drive (with the analog
header)
1.
If you would like to use the physical playback buttons on your CD-ROM drive
to control audio CD playback on your PC, you need to connect the analog
cable between the CD-ROM drive and the audio card. Otherwise, you can
just skip this section and use ordinary digital CD playback on Windows.
2. Connect the CD-IN 4-pin header to the analog output on the back of your
CD/DVD-ROM drive with the audio CD cable which is usually included with
CD/DVD-ROM drive.