i thought exoplayer defaults to support passthrough but maybe not in all cases. googling shows plex having a passthrough issue with exoplayer. My shield + receiver handles EAC3 fine (thats what all my video are encoded with). I do plan on looking into a separate audio passthrough toggle, but i'd like to get transcoding working first. The new chromecast only supports eac3/ac3 passthrough and one of my tv has issues with some eac3 files.
I actually dont think i've tested multi library for emby so i'll have to give that a shot
kodi definitely has its own audio codecs that the device doesnt natively supports. It can playback almost anything. For emby im guessing its just using the pass through which for some reason isnt auto enabled for you. A manual setting for passthrough (and therefore quasi tv forcing it on) might resolve that. The transcoding would also handle it.