So Plex on the Roku Ultra is doing some curious things with the audio modes: "night", "leveling", and "off".
When I have night mode on, any Dolby eAC3/AC3 tracks are converted to AAC. When night mode is off, any AAC 5.1 tracks are converted to AC3 5.1. AAC mono and 2.0 tracks are left untouched. Why is it doing this?
(As an aside, it seems Roku cannot decode AAC 7.1 at all and will convert to aac stereo if night mode on or eAC3 7.1 if off. Is it not compatible or something?)
The audio 'modes' can't deal with multichannel compressed/encoded bitstream formats so plex is converting to a format that it can use. Also, I don't think most (any?) roku models handle more than 2 channel AAC so plex converts to AC3 to preserve multichannel when you aren't using a mode than needs to alter it.