opensuse 42.1
Does opensuse have a program that reads the volume level of a directory full of audio files, and adjusts the volume setting so that they are roughly equal? The files are a mix of M4A and MP3 formats.
I have researched a bit. ffmpeg seems one solution; it is rather complicated to use and create a script that would do the work. “normalize” only works on MP3 files.
Take a look at the Digital Audio Editor “audacity”.
It has a “normalize” feature which will allow you to set the audio level to a chosen value, optionally normalizing stereo channels independently, and to remove any DC offset.
There is a batch processing ability, audacity chose to call that “chains”, so it’s not perhaps immediately obvious.
You might want to take a look at the user manual first… 
> There is a batch processing ability, audacity chose to call that “chains”, so it’s not perhaps immediately obvious.
>You might want to take a look at the user manual first…
>
It looked promising… until I saw there was no way to save an M4A file using a “chain.” I’ll ask at their user forum.
Ah… OK, sorry about that. I don’t use M4A myself (just flac and the odd mp3), and had rather foolishly assumed all export options would be available. Their user forums are quite good, so hopefully you’ll find a solution.
Just a further thought…
Does the option to export to “M4A (AAC) Files (FFmpeg)” appear under “File → Export Audio…”
You may need to install the version of audacity from the “packman” repository rather than the normal openSUSE one. (I believe the openSUSE version of the FFmpeg library is different to comply with copyright issues).
If you do switch to packman then it’s probably best to do a full vendor change to that. (Details https://en.opensuse.org/SDB:Vendor_change_update ).
You could also look at soundKonverter. It is a KDE front end for a large range of audio tools (e.g. ffmpeg) and simplifies audio file conversion and Replay Gain calculations.