Two separate problems here:
- subtitles.
- audio.
Subtitle support in mpv was just added 3 days ago but I’m not sure it covers all the formats. No idea but we can look into this.
Audio could be several problems: Volume is 0%. Audio is routed to a non-plugged in output (common with analog out and HDMI/DP monitors plugged in without speakers!) or there is an unsupported audio codec in the media (AAC).