Yup.
You mean background music? It is changeable from the settings menu of the web player.
In Settings -> Plex/Web.
I second this request. I like having the theme music there, but it would be nice to silence it for some shows, either because they are annoying or maybe I don't want to hear the theme music right then. Maybe just add a small theme mute button, that is saved per-show.
I don't want to disable the feature in general, sometimes I just don't want to hear the theme right then (e.g. I'm listening to music, so I don't want to mute my whole machine).
2022 clean-up: duplicate (this thread is obviously much older but seems to have been abandoned; there’s another thread covering this suggestion, considering other aspects of background playback of theme music as well)