Hi Everyone! I have been looking into this for a while and I was searching around on and off, but I came across a file
a few days ago that sparked a few things in my head and there is a way to get external audio files to work on plex!
The thing is I haven’t figured out all the details but I can tell you what I do know. The AC3 File format for the external audio does not get seen by plex indeed BUT I do have a movie with an external audio file that is seen by plex that is M4A Format and I am able to extract the info from the file that tells me the codec used and such on the file. I am testing converting an AC3 file in the attempt to get the right formula that makes the audio work like that other one I mentioned (which I did not make or encode or such btw). Here is the information from the working file:
General
Complete name : Movie Name.m4a
Format : MPEG-4
Format profile : Base Media / Version 2
Codec ID : mp42 (M4A /mp42/isom)
File size : 191 MiB
Duration : 2 h 4 min
Overall bit rate mode : Variable
Overall bit rate : 214 kb/s
Audio
ID : 1
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : HE-AAC / LC
Codec ID : 40
Duration : 2 h 4 min
Bit rate mode : Variable
Bit rate : 213 kb/s
Maximum bit rate : 268 kb/s
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 kHz / 24.0 kHz
Frame rate : 23.438 FPS (1024 spf)
Compression mode : Lossy
Stream size : 190 MiB (100%)
Writing library : Nero AAC codec 1.5.4.0
Encoding settings : -q 0.30
Please let me know if this helps or if you figure something out that works by using this information. Thanks! 
