Asus Xonar U3 Quick Start Manual page 6

For Digital Speakers
SPDIF Out
Plug the optical TOSLINK adapter into the S/PDIF-Out port. Then, connect your
decoder's S/PDIF In port to the optical adapter with a TOSLINK optical cable.
The TOSLINK optical cable is purchased separately.
Connecting headphone / microphone
6 Xonar U3 Quick Start Guide