Use mpv for downmixing to stereo

It supports different input channel configurations and does not require
a long string of magic numbers.

docs/media/index.md

@@ -12,10 +12,10 @@
     :::shell
     for i in *.flac; do tags=$(metaflac --export-tags-to=- $i | grep -E '^MUSICBRAINZ_'); metaflac --remove-all-tags $i; metaflac --import-tags-from=- $i <<< $tags; done
 
-### Downmix 5.1 to 2.0
+### Downmix 5.1/7.1 to 2.0
 
     :::shell
-    sox in.wav out.wav remix -m 1v0.3694,3v0.2612,5v0.3694 2v0.3694,3v0.2612,6v0.3694 norm
+    mpv --oac=flac --audio-channels=stereo --oacopts=compression_level=0 --o=outfile.flac infile.flac
 
 ### Record pulseaudio device to flac