Use something you know supports 5.1, like HTML5 AAC Audio Playback Tests - Multichannel. If that test doesn't work, and Chrome is using pulse than you know the issue is with Pulse Audio and you must dive deeper.
Next play something in Chrome, like this long ass Infected Mushroom album which is 100% good. Leave it playing, and while it's playing run pacmd list-sink-inputs
. It should show you something like this
1 sink input(s) available.
index: 381
driver: <protocol-native.c>
flags: START_CORKED
state: RUNNING
sink: 1 <alsa_output.usb-Burr-Brown_from_TI_USB_Audio_CODEC-00.analog-stereo.2>
volume: front-left: 65536 / 100% / 0.00 dB, front-right: 65536 / 100% / 0.00 dB
balance 0.00
muted: no
current latency: 56.23 ms
requested latency: 23.22 ms
sample spec: s16le 2ch 44100Hz
channel map: front-left,front-right
Stereo
resample method: (null)
module: 14
client: 410 <Chromium>
properties:
application.icon_name = "chromium-browser"
media.name = "Playback"
application.name = "Chromium"
native-protocol.peer = "UNIX socket client"
native-protocol.version = "31"
application.process.id = "2148"
application.process.user = "ecarroll"
application.process.host = "x230"
application.process.binary = "chromium-browser"
window.x11.display = ":0.0"
application.language = "en_US.UTF-8"
application.process.machine_id = "0275a7cffa4379527ee564ba552469a2"
application.process.session_id = "c2"
module-stream-restore.id = "sink-input-by-application-name:Chromium"
Going through this, this is the index of the sink
,
sink: 1 <alsa_output.usb-Burr-Brown_from_TI_USB_Audio_CODEC-00.analog-stereo.2>
And, this is the data the sink-input is currently receiving, if this isn't right then the source you're playing isn't right and it's not your problem.
channel map: front-left,front-right
Stereo
It should read like this if the music is 5.1
channel map: front-left,front-right,front-center,lfe,rear-left,rear-right
Surround 5.1
Now run this.
pacmd list-sinks | grep -i "channel\|index"
This should show you all of the sinks (channels) that can accept sound. Pay attention to the one with the corresponding index, if it shows you something like this.
channel map: front-left,front-right
Then you /know/ that pulse will be downmixing the channel to stereo. Otherwise, we'll need more information.