Asus XONAR U5 SI User Manual page 20

4. I can't hear any sound from the S/PDIF output
(1). Ensure that you have enabled S/PDIF output in the main settings of Sonic
Studio.
(2). Ensure that you have connected the correct S/PDIF output jack in the card to
the decoder's (AV receiver) S/PDIF input jack.
(3). You may need to select the correct input and mode of your decoder or AV
receiver.
(4). If you are using 192KHz PCM output, make sure that your decoder can
support 192KHz decoding. Try changing it to 44.1K or 48KHz first.
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ASUS Xonar U5 SI User Guide