I have an LG DVD burner and a CD rom drive. Both drives need a sound cable connected to the MOBO to get sound from the speakers.
I have just (yesterday) replaced the CD Rom drive with a Pioneer DVD Rom drive. Although it came with a sound cable I didn't connect the sound cable, but when I tested it with a DVD Movie I get full sound from the speakers?
It's the wat they tend to work digitally nowadays, cable is used for analogue but most don't need it on modern computers.
It's the way the sound board interconnects. I haven't used a cable for at least five years, you will probably see that if you turn down or mute the CD volume control on the sound card software nothing happens, as that is normally analogue via cable.