Pulseaudio sink inputs have incorrect channels for surround audio

Using Firefox from Mozilla PPA on Ubuntu 22.04, testing the ident samples available at https://www2.iis.fraunhofer.de/AAC/multichannel.html, the Pulseaudio sink inputs are always created as stereo, even for >2 channel cases.

The snap-based Firefox distributed by Canonical with Ubuntu 22.04 does not have this problem. There, channel counts and channel maps for the sink inputs are correct.

Example sink input from mozillappa (100.0+build2-0ubuntu0.22.04.1~mt1), playing the first example from the above Fraunhofer page:

Sink Input #841
    Driver: protocol-native.c
    Owner Module: 11
    Client: 1732
    Sink: 98
    Sample Specification: float32le 2ch 44100Hz
    Channel Map: front-left,front-right
    Format: pcm, format.sample_format = "\"float32le\"" format.rate = "44100" format.channels = "2" format.channel_map = "\"front-left,front-right\""
    Corked: no
    Mute: no
    Volume: front-left: 65536 / 100% / 0.00 dB, front-right: 65536 / 100% / 0.00 dB
            balance 0.00
    Buffer Latency: 73628 usec
    Sink Latency: 75028 usec
    Resample method: copy
    Properties:
        media.name = "AAC Multichannel Playback Test"
        application.name = "Firefox"
        native-protocol.peer = "UNIX socket client"
        native-protocol.version = "35"
        application.process.id = "115748"
        application.process.user = "xxx"
        application.process.host = "xxx"
        application.process.binary = "firefox"
        application.language = "en_CA.UTF-8"
        window.x11.display = ":0"
        application.process.machine_id = "50cf6dd259f048ec85009fa2dcee6716"
        application.process.session_id = "c1"
        application.icon_name = "firefox"
        module-stream-restore.id = "sink-input-by-application-name:Firefox"

For comparison, here is an example sink input created by the snap-based Firefox that comes out-of-the-box with Ubuntu 22.04, showing the expected channel configuration:

Sink Input #918
 Driver: protocol-native.c
 Owner Module: 11
 Client: 2486
 Sink: 160
 Sample Specification: float32le 6ch 44100Hz
 Channel Map: front-left,front-right,front-center,lfe,side-left,side-right
 Format: pcm, format.sample_format = "\"float32le\"" format.rate = "44100" format.channels = "6" format.channel_map = "\"front-left,front-right,front-center,lfe,side-left,side-right\""
 Corked: no
 Mute: no
 Volume: front-left: 65536 / 100% / 0.00 dB, front-right: 65536 / 100% / 0.00 dB, front-center: 65536 / 100% / 0.00 dB, lfe: 65536 / 100% / 0.00 dB, side-left: 65536 / 100% / 0.00 dB, side-right: 65536 / 100% / 0.00 dB
         balance 0.00
 Buffer Latency: 105850 usec
 Sink Latency: 52961 usec
 Resample method: copy
 Properties:
  media.name = "AAC Multichannel Playback Test"
  application.name = "Firefox"
  native-protocol.peer = "UNIX socket client"
  native-protocol.version = "33"
  application.process.id = "161759"
  application.process.user = "xxx"
  application.process.host = "xxx"
  application.process.binary = "firefox"
  application.language = "en_CA.UTF-8"
  window.x11.display = ":0"
  application.process.machine_id = "50cf6dd259f048ec85009fa2dcee6716"
  application.process.session_id = "41"
  application.icon_name = "firefox"
  module-stream-restore.id = "sink-input-by-application-name:Firefox"