Just a simple personal solution, used with ws-scrcpy
I use Flutter to write an App myself,
use inappwebview to open the NetrisTV/ws-scrcpy webpage (with viewport to improve resolution),
and then use the headless (inappwebview) background to open the audio webpage

The following is a simple method to play mobile phone audio on the web page

1. Start bluenviron/mediamtx

   docker run --name=mediamtx \
   --env=MTX_WEBRTCADDITIONALHOSTS=<External public network IP>,<Internal network host IP> \
   -p 1935:1935 \
   -p 8189:8189  \
   -p 8189:8189/udp \
   -p 8554:8554 \
   -p 8888:8888 \
   -p 8889:8889 \
   -p 8890:8890/udp  \
   bluenviron/mediamtx

2. Start Genymobile/scrcpy and use ffmpeg push the audio to mediamtx
   [/config/.android] <- Used for adb authorization to avoid having to confirm every time you operate the phone
   The 3000 port is kasmVnc. I originally used this webpage to play audio, but the network usage was relatively high.

   docker run --name=mediamtx-scrcpy-audio \
   --env=SCRCPY_IP=<mobile phone ip:5555> \
   --env=RTSP_SERVER=rtsp://mediamtx:8554/myaudio1 \
   --volume=./android:/config/.android:ro  \
   -p 3000:3000 \
   ghcr.io/otaku840726/mediamtx-scrcpy-audio:latest

3. Open the audio webpage
   First use the internal network to confirm that the audio is normal. The external network needs to enable 8189/udp to use webrtc normally.
   kasmVNC is about 400k/s, Mediamtx webrtc is about 30k/s
   But of course there will be differences in sound quality. You can also use the Docerfile to rebuild it yourself. There is bitrate/compression_level/frame_duration in root/entrypoint.sh to improve it.
   mediamtx-webrtc: http://mediamtx:8889/myaudio1
   kasmVNC: http://mediamtx-scrcpy-audio:3000