"External player" with local media?



I'm trying to understand what this app is doing. I have a video I'm trying to play on my phone that has EAC3 audio. With default settings, the Plex android app tells me that it requires a media server to play. If I go into the settings and select "External player", it will play the file in Exoplayer (without sound). Isn't Exoplayer the default internal player for the Plex app?

Additionally, if I attempt to play any file from my PC media server while the "external player" option is enabled, I get a prompt to select the app I want to use. Why is this behavior different from local playback? I'd much rather use an external player for local media, since VLC or MX Player can handle EAC3 audio and exoplayer can't! That's aside from the "file.mkv" bug that usually causes the app to crash.

Is there any way to reverse this behavior? Or should I just use a different app entirely for local media and casting?