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Force transcoding ac3 to AAC (but just audio, not video)

Hi everyone, I'm trying to solve an issue that is bugging me from a while.

PROBLEM: Samsung natively can play AC3 but on a lot of files (mainly mkv) central channel is very low when using tv speakers. voices are really low even at 80-90% of the tv volume. I've tried to understand why some files works ok and other no, but I cannot find a reason. I've read around on other forums and other people have this same issue. The problem is the same if you play the file from a USB with the native samsung player, so is not a Plex App problem.

SOLUTION (that was working for a while): I've edited the Samsung profile (making a copy) and deleted AC3 from the list of direct play supported codec. Then chose "direct streaming" from the player. Video got passed through, while audio was transcoded to AAC. perfect, problem solved: audio volume was ok and CPU on my old MacBook Pro (the server) could do the job.

But at a certain point this hasn't worked anymore. Don't know if this is a player problem or a server problem, but I get different results depending on the file. Mostly I get both audio and video transcoded. Sometimes I get video transcoded and audio passed through. I never get video passed through and audio transcoded. The problem is that the server cannot do video transcoding, it is not powerful enough and I get lot of buffering, also lowering the quality. Then: why transcode video if the problem is just the audio?

here is my configuration:
TV: UE55H7000 2014 model
Network: LAN
Plex App version: 2.008
PMS: 1.8.1.4139

File I'm using as a test:

Media

Video Resolution 1080p
Duration 1:37:04
Bitrate 11444 kbps
Width 1920
Height 1080
Aspect Ratio 1.78
Container MKV
Video Frame Rate 24p
Video Profile high

Codec H264
Bitrate 9844 kbps
Bit Depth 8
Chroma Subsampling 4:2:0
Color Range tv
Color Space bt709
Frame Rate 23.976 fps
Height 1080
Level 4.1
Profile high
Ref Frames 4
Scan Type progressive
Width 1920

Codec AC3
Channels 5.1
Bitrate 640 kbps
Language Italiano
Audio Channel Layout 5.1(side)
Sampling Rate 48000 Hz

Transcoding status (from PMS browser app)
Video: transcoding H264 to H264
Audio: Transcoding AC3 to AC3

Edited profile

<?xml version="1.0" encoding="utf-8"?>
<Client name="Samsung">
    <!-- Author: Plex Inc. -->
    <TranscodeTargets>
        <VideoProfile protocol="hls" container="mpegts" codec="h264" audioCodec="ac3" context="streaming">
            <Setting name="VideoEncodeFlags" value="-x264opts ref=4:bframes=2:cabac=1"/>
        </VideoProfile>
        <SubtitleProfile container="srt" codec="srt" context="all" />
        <MusicProfile container="mp3" codec="mp3"/>
        <PhotoProfile container="jpeg"/>
    </TranscodeTargets>
    <DirectPlayProfiles>
        <VideoProfile container="mp4" codec="h264,mpeg4" audioCodec="aac,ac3,eac3,mp3,pcm" subtitleCodec="srt,ssa,ass"/>
        <VideoProfile container="asf" codec="wmv3,wmv3,vc1" audioCodec="wmav2,wmav2,wmapro,wmavoice,pcm" subtitleCodec="srt,ssa,ass"/>
        <VideoProfile container="avi" codec="h264,msmpeg4v3,mpeg4,mjpeg" audioCodec="mp3,eac3,dca,pcm" subtitleCodec="srt,ssa,ass"/>
        <VideoProfile container="mkv" codec="h264,mpeg4,mjpeg,mpeg2video,vc1,hevc" audioCodec="mp2,mp3,dca,aac,pcm,vorbis" subtitleCodec="srt,smi,ssa,ass,subrip"/>
        <VideoProfile container="mpeg" codec="h264,mpeg1video,mpeg2video" audioCodec="mp2,mp3,ac3,eac3,aac,pcm" subtitleCodec="srt,ssa,ass"/>
        <VideoProfile container="flv" codec="h264" audioCodec="aac,ac3,eac3,mp3,pcm" subtitleCodec="srt,ssa,ass"/>
        <VideoProfile container="mpegts" codec="h264,mpeg2video,vc1" audioCodec="mp2,mp3,ac3,eac3,dca,pcm" subtitleCodec="srt,ssa,ass"/>
        <VideoProfile container="wtv" codec="mpeg2video" audioCodec="ac3,eac3,aac,pcm"/>
        <VideoProfile container="3gpp" codec="h264,mpeg4" audioCodec="aac,he-aac"/>
        <MusicProfile container="mp4" codec="aac"/>
        <MusicProfile container="mp3" codec="mp3"/>
        <MusicProfile container="flac" codec="flac" />
        <MusicProfile container="wma" codec="wmav2" />
        <MusicProfile container="asf" codec="wmav2" />
        <MusicProfile container="wav" codec="flac,pcm" />
        <PhotoProfile container="jpeg,gif,bmp,png"/>
    </DirectPlayProfiles>
    <CodecProfiles>
        <VideoAudioCodec name="ac3,wmav2,dca,aac,mp3">
            <Limitations>
                <UpperBound name="audio.channels" value="6"/>
            </Limitations>
        </VideoAudioCodec>
        <MusicCodec name="aac">
            <Limitations>
                <UpperBound name="audio.channels" value="2"/>
            </Limitations>
        </MusicCodec>
        <MusicCodec name="mp3">
            <Limitations>
                <UpperBound name="audio.bitrate" value="320"/>
                <UpperBound name="audio.channels" value="2"/>
            </Limitations>
        </MusicCodec>
    </CodecProfiles>
</Client>
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