One cool thing about mpv is that it can transcode audio encoded in different surround sound formats (AAC, FLAC, Atmos, etc.) into regular Dolby Digital for use with old pre HDMI AVRs using SPDIF. AFAIK it's the only player that can do this natively without any additional work on the OS level.
I'll just add this here but if you want this kind of transcoding for the entire system rather than just mpv you can use dcaenc with its virtual 6 channel input encoding everything to DTS. I send the encoded audio to the AVR using a cheap TI PCM27XX sound card with optical output.