With PulseAudio, we can send a RTP stream over multicast UDP. Here is the bash commands to setup a sink where anything sent to it gets broadcasted to the multicast address 224.0.0.56 at port 46416.

pacmd load-module module-null-sink sink_name=RTP
pacmd update-sink-proplist RTP device.description=RTP
pacmd load-module module-rtp-send \
  source=RTP.monitor \
  destination_ip=224.0.0.56 \
  port=46416

Then in pavucontrol, you can push the audio of any application to the RTP sink we just created.