--audio-spdif=<codecs>
List of codecs for which compressed audio passthrough should be used. This works for both classic S/PDIF and HDMI.
Possible codecs are ac3, dts, dts-hd, eac3, truehd. Multiple codecs can be specified by separating them with ,. dts refers to low bitrate DTS core, while dts-hd refers to DTS MA (receiver and OS support varies). If both dts and dts-hd are specified, it behaves equivalent to specifying dts-hd only.
In earlier mpv versions you could use --ad to force the spdif wrapper. This does not work anymore.
Warning
There is not much reason to use this. HDMI supports uncompressed multichannel PCM, and mpv supports lossless DTS-HD decoding via FFmpeg's new DCA decoder (based on libdcadec).