No se si será el caso pero lo he sacado de una página y aparte he leído que los drivers están desactualizados, tampoco se seguro si se aplica a la tuya.
Update 20200127: The article below is in regard to the Asus Xonar DX and similar cards that support Dolby Digital Live!, which is responsible for taking 5.1 PCM audio and converting it in real-time (via the Toslink / SPDIF / optical port on the card) to 5.1 Dolby Digital surround sound and outputting it to your stereo receiver. If your Asus Xonar card does not have the Dolby Digital Live! feature, then the games you play on the PC will not be in true 5.1 surround sound. In this case, your stereo receiver will up-convert the signal to another technology such as DTS or Dolby Pro Logic. Even so, this article explains quite a bit, so feel free to read it even if you don't have a Xonar DX model.
Si te digo la verdad con una tarjeta sound blaster y el óptico tuve problemas similares y al final opté por las analógicas y cero problemas