ryecoaaron, thanks for your response and inquiry.
Music folders has no sound. Movies, of type mkv, which I understand is supported, give a codec error. Videos that I've recorded with my smartphone have sound and play properly, and Photos show up OK.
<edit> Actually, music folder songs never start - the count up timer just stays at 00:00, then advances to the next song, and does the same thing. The album cover is there, and the song list is there, and the length of time the song occupies is there, but it never plays. At the bottom, there are <<- <stop> ->> buttons so it's as if it's trying to play but nothing happens, then it just advances. On the Plex forum, see other people have had this problem too.
I just didn't want to trouble OMV support with this, as it's clearly a Plex problem, so just thought I'd do better to do a Plex install and then could work with Plex support without them finger-pointing at OMV being the problem.