No 5.1 surround from Plex web player on Mac



I don't know when it happened exactly, but I'm no longer getting 5.1 surround when playing back videos with 5.1 audio.

I'm running macOS 10.12.6 with the latest ( media server. When I play the videos with VLC, I get 5.1 audio, but I don't when I play them via Plex in Safari.

Any help would be appreciated.