Downmixing audio channels

ls -1 *.mp4 | xargs -I{} ffmpeg -y -i {} -ac 2 -c:v copy /path/to/out/{}

Downmixes multi-channel audio into stereo. Reduces the chances of weird things happening during playback.

-c:v copy is important for performance as it means the video isn't re-encoded (10x improvement in speed).