Should LinguaLibre normalize the loudness of recordings to EBU R 128, so that pronunciations are perceived equally loud irrespective of user microphones?
ffmpeg can do this, either if you call it directly (rather painful), or via the ffmpeg-normalize wrapper script. It’s also possible to embed metadata with loudness parameters, which some (but not all) players recognize. But in the context of LinguaLibre, which does not use a lossy audio compression, it might be best to normalize loudness on the server at preprocessing time.
See discussion in the Lingua Libre chatroom for the thoughts of others.