ReplayGain is a proposed technical standard published by David Robinson in 2001 to measure and normalize the perceived loudness of audio in computer audio formats such as MP3 and Ogg Vorbis. It allows media players to normalize loudness for individual tracks or albums. This avoids the common problem of having to manually adjust volume levels between tracks when playing audio files from albums that have been mastered at different loudness levels.
Bear in mind the entry's "Portable Media Players" section is about hardware players; for software options, look a little further down...
Then there's loudgain...
loudgain is a versatile ReplayGain 2.0 loudness normalizer, based on the EBU R128/ITU BS.1770 standard (-18 LUFS) and supports FLAC/Ogg/MP2/MP3/MP4/M4A/ALAC/Opus/ASF/WMA/WAV/WavPack/AIFF/APE audio files. It uses the well-known mp3gain commandline syntax but will never modify the actual audio data. Just what you ever wanted: The best of mp3gain, ReplayGain 2.0 and Linux[!] combined.
Unfortunately, loudgain doesn't seem to be Windows compatible -- although there are instructions to run it under a Windows 10 bash shell; moreover, development appears to have stalled somewhat (v0.6.8 released 2019-09-06), so hope isn't high for a proper Windows release.
Finally, Hydrogen Audio forum has an old topic with instructions on how to make AACGain (original 2010 v1.9 available from RareWares; a more recent v2.0 exists at Github, which is a fork of the former) work with MP3Gain (and which will perhaps still work with more updated wxMP3gain) by simply replacing executables (untested!) -- something that may be worthwhile for processing MP4s:
