https://askubuntu.com/questions/17791/can-i-downmix-stereo-audio-to-mono 1.Find the name of your audio sink by running
pacmd list-sinks | grep name:
2.Then run this command (taking care to remove the angled brackets from the result of previous command):
pacmd load-module module-remap-sink sink_name=mono master=NAME_OF_AUDIO_SINK_GIVEN_BY_PREVIOUS_COMMAND channels=2 channel_map=mono,mono
or add