Add notes about how to downmix audio

notes/ffmpeg-downmixing-audio.md

@@ -0,0 +1,15 @@
+---
+title: Downmixing audio channels
+tags:
+  - Media
+emoji: 🎵
+link: https://trac.ffmpeg.org/wiki/AudioChannelManipulation
+---
+
+```bash
+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).