Sonos Amp: No 5.1 only Stereo 2.0

  • 11 February 2023
  • 4 replies

Userlevel 3


I need some help please to troubleshoot why I am only getting “Stereo 2.0” sound on my Sonos Amp, please.

Physically I have an old Panasonic TX-P50S30B plasma connected to the Sonos Amp over HDMI (via ARC). Connected to the Amp are two floorstanding speakers. Source material is fed in via Plex using a Google Chromecast Gen 3 again via HDMI into the Panasonic TV. This works well but I only get “Stereo 2.0” sound reported in the Sonos app, even when I try to play AC3 5.1 material from Plex.

So obviously _something_ in my supply chain is down mixing to stereo but I am not sure what.

  1. I’ve tried connecting older Sonos Play 1’s as surrounds to the Amp. These work but again “Stereo 2.0” only despite the actual 4.0 configuration.
  2. The Chromecast and my TV are set to “Auto” for audio.
  3. My TV definitely passes 5.1 audio via optical as I have used that before with an older amp, prior to the Sonos Amp.
  4. Thinking that perhaps my TV is only passing stereo via HDMI, I connected a Sonos Optical adaptor to the Panasonic into the Amp instead. But again, I only get Stereo 2.0.

I have zero controls for audio via Plex (via PC or the app), so I assume it is pushing AC3 5.1 when possible. But it’s easy to go around in circles here (Plex support blame Chromecast/Sonos and vice versa etc.)

The Sonos Amp itself _does_ virtualise Stereo 2.0 into 4.0 in my configuration (at least it sounds reasonably good) but it’s not doing proper surround effects if I use a test track.

Any clues how to troubleshoot this next or what I am missing here?

Many thanks in advance!


Best answer by GuitarSuperstar 14 February 2023, 17:23

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4 replies

Does the TV offer a “Passthrough” audio option?

As HDMI connections are established there is a lot of back and forth negotiation as both boxes attempt to work out a compatible set of features to enable. Sometimes this negotiation goes off the track. Try removing HDMI connections to the TV, then reboot both AMP and the TV. After they have completed their reboot, reconnect the HDMI. While this is not the ultimate panacea, it sometimes works through an issue. Make sure that CEC (Panasonic calls it “Viera Connect”) is enabled on the TV because some TV’s will not send premium audio formants unless CEC is enabled.

Userlevel 3

 Thank you @buzz that’s helpfu.

I’ve done more experimenting and reading on other forums, and am slowly concluding this is an issue with the Panasonic TV passing through audio properly - it will only pass through stereo via PCM, not DD 5.1 or any surround format via HDMI. So I need to try an HDMI Audio Extractor gizmo and feed the optical out from that into the Sonos Amp - I suspect this will work but am waiting for the hardware to try it.

SPDIF Selection

Selects the initial setting for digital audio output signal from DIGITAL
AUDIO OUT and HDMI2 (ARC function) terminals [Auto] / [PCM]
[Auto] : Dolby Digital Plus, Dolby Digital and HE-AAC (48 kHz) multi-
channel are output as Dolby Digital Bitstream.
DTS is output as DTS.
MPEG is output as PCM.


In fact, I remembered now that the configuration I previously had that “worked” with 5.1 sound and my old receiver was with a PS3/DVD wired optical out directly into the receiver, bypassing the TV.

The TV has a setting under “Sound” for S/PDIF which I have set to “Auto” (or can be set to “PCM” which is not what I want). But this setting isn’t visible with an HDMI source (i.e. the Chromecast) and I don’t think it makes any difference here practically.

Viera Connect is enabled and otherwise the Amp works well, but only Stereo 2.0.

Userlevel 7

@sonos_remerson I can pretty much confirm that your model TV cannot pass through 5.1 audio from any external media devices. Your model TV is from 2011. I have a Panasonic ST60 from 2013 connected to a Sonos Arc. I was unable to get any 5.1 audio other than from the TV’s native VUDU app and playing 5.1 video files from a USB flash drive connected to the TV. Everything else played in Stereo PCM. I ended up having to get an HDFury Arcana eARC adapter. This allows all my external media devices to bypass my TV and send audio directly to the Arc resulting in 5.1 audio.

Userlevel 3

@GuitarSuperstar Thank you for confirming - that’s really helpful.

And more helpful still was you reminding me that the TV can play via USB (which I’d forgotten all about in this age of streaming convenience!). Indeed, if I try to play DD 5.1 material from USB, it sends DD 5.1 out to the Sonos Amp via HDMI ARC just fine. To me, that points squarely at the HDMI inputs into the Panasonic being limited to Stereo 2.0 only exactly as you say.

Oh if only Panasonic wrote that in the manual, it would have saved me so much trouble (but I probably would have learned less).

I’m still awaiting my HDMI audio extractor box and will confirm once tested.

I read a lot of good things about the HDFury Arcana - it seems to be the “gold standard” for this issue and makes perfect sense with the Sonos Arc. But I think it’s overkill for me with the Amp (and more costly) as the Amp is limited to DD 5.1 / AC3 and plain HDMI ARC has enough bandwidth for that, so I’ve gone for a cheaper “no-frills” extractor box.