Attempting to backup the various media filetypes has become a nightmare, just the way WIPO attorneys want it. They own the media, not us. Of course, we do own things in 2025, but they've made it nearly impossible to keep a steady format that will play across devices. Even now, these turn out to be MP3's. MP4's now contain so many different codecs that sometimes they will play, other times they do not. Typically an H264 encoding with AAC audio will still go on nearly anything.
First get a list of the installed applications, to see if any apps are necessary.
$ pacman -Qet
sample WEBM with OPUS audio
Suppose a standard download in 720p, no particular audio spec.
$ yt-dlp -S res:720 "https:foo"
So we now have this file "foo.webm", which is in a WEBM container, not an MP4 container, but we only want the audio. First we check it.
$ ffmpeg -i foo.webm
Input #0, matroska,webm, from 'foo.webm':
Metadata:
COMPATIBLE_BRANDS: iso6av01mp41
MAJOR_BRAND : dash
MINOR_VERSION : 0
ENCODER : Lavf61.7.100
Duration: 01:20:00.87, start: 0.000000, bitrate: 910 kb/s
Stream #0:0: Video: av1 (libdav1d) (Main), yuv420p(tv, bt709), 1280x720, SAR 1:1 DAR 16:9, 24 fps, 24 tbr, 1k tbn (default)
Metadata:
HANDLER_NAME : ISO Media file produced by Google Inc.
VENDOR_ID : [0][0][0][0]
DURATION : 01:20:00.834000000
Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
Metadata:
DURATION : 01:20:00.868000000
So the OPUS filetype is the audio. Thus, to avoid re-encoding unintentionally, we should just pull it out as an OPUS file without additional actions.
$ ffmpeg -i foo.webm -vn -acodec copy fooaudio.opus
We'd like to leave it an OPUS, but OPUS won't play on anything rolling. So, let's say 256Kb or 320Kb MP3 should do it. That's re-encode 1. We can use, eg Ocenaudio to edit the OPUS to whatever we need and save it as an MP3. I might want to touch it up further to change the speed with sox, which makes a second encode. I might turn up the volume a touch to make sure it overcomes that.
$ sox -v 1.1 -i fooaudio.mp3 C 320 fooaudio90.mp3 speed 0.90
