hook.py

#!/usr/bin/python3

import sys
import os
import re

args = sys.argv
layout = {
    "stereo": "2",
    "5.1": "6"
}

for i in range(len(args)):
    if args[i] == "-filter_complex" and ":ochl=" in args[i+1]:
        fc = re.search("^\[([^\]]+)\].*:ochl='([^']+)'.*:osr=([0-9]+)", args[i+1])

        if fc.group(2) in layout:
            args.pop(i)
            args.pop(i)

            for ii in range(i, len(args)):
                if args[ii] == "-map":
                    args[ii+1] = fc.group(1)
                    break

            for ii in range(i, len(args)):
                if "-codec:" in args[ii]:
                    stream = re.search("-codec:(.+)", args[ii]).group(1)
                    args.insert(ii+2, f"-ac:{stream}")
                    args.insert(ii+3, layout[fc.group(2)])
                    args.insert(ii+4, f"-ar:{stream}")
                    args.insert(ii+5, fc.group(3))
                    args.insert(ii+6, f"-filter:{stream}")
                    args.insert(ii+7, "acontrast=100")
                    break

        break

os.execv("/usr/lib/plexmediaserver/PlexTranscoder", args)

override.conf

[Service]
ExecStartPost=+/var/lib/plexmediaserver/pre-hook.py

pre-hook.py

#!/usr/bin/python3

import os

if not os.path.islink("/usr/lib/plexmediaserver/Plex Transcoder"):
    os.replace("/usr/lib/plexmediaserver/Plex Transcoder", "/usr/lib/plexmediaserver/PlexTranscoder")
    os.symlink("/var/lib/plexmediaserver/hook.py", "/usr/lib/plexmediaserver/Plex Transcoder")