FFMPEG - Audio missing after combining video
You have to mix both audios, else ffmpeg will pick one from among the inputs.

ffmpeg -y -threads 4 -loglevel quiet -ss 0 -t 220.162 -i left.webm 
-ss 0 -t 220.162 -i right.webm -filter_complex "[0]scale=320:-1,pad=2*iw:2*ih:0:120[left];
[1]scale=320:-1[right];[left][right]overlay=main_w/2:120,scale=640:480;[0][1]amix" -shortest output.mp4