A lot of older computers didn't have front audio ports on the case. Instead, they were built into the CD/DVD drive and had a cable that went back to a sound card or integraded audio of the motherboard.
Some old CD or DVD drives also had other pins on the back for jumpers to control master/slave/etc. on IDE/PATA drives and also early SATA drives that were IDE/PATA with a built-in adapter.