Plex media server requires the DLNA server option to be on for it to be discovered by your network music player. This can sometimes become disabled after a software update for Plex.
In order to correct the issue, turn the DLNA option on. If it is already on, turn it off and save changes. Turn the option back on, and save changes again.
If you continue to experience this after enabling the DLNA option, please contact our support team.