The short answer
ReplayGain writes a note in the file and leaves the sound alone, and an MP3Gain-style gain change edits a volume setting inside each MP3 and does not re-encode the sound. Re-encoding rewrites the sound itself. Here they are side by side.
| Method | Where the change lives | Changes the audio data? | Works in every player? | Smallest step |
|---|---|---|---|---|
| ReplayGain tags | A number saved in the file's tags | No | No, only in players that read the tag | Very fine, because the tag holds an exact value |
| MP3Gain-style gain change | The volume setting inside each MP3 frame | The setting changes, but the sound is not re-encoded, and the change can be reversed | Yes | 1.5 dB |
| Re-encoding | A new copy of the audio | Yes, it is decoded and encoded again | Yes | Any amount |
ReplayGain: a note in the tags
ReplayGain measures how loud a song sounds to a listener, then saves the result in the file's tags as a track gain, and sometimes an album gain, and a player that understands the tag turns the song up or down while it plays. The audio data in the file does not change.
The target level for almost every ReplayGain tool is 89 dB, measured as pink noise played back on a calibrated system, and that number matters less than it sounds, because it is only a reference point. What matters is that the tag does nothing in a player that ignores it.
MP3Gain: changing the setting inside the file
Every frame of an MP3 carries a setting called global gain, a whole number from 0 to 255. When a player decodes the frame, it uses that number to scale the sound. Add one to the number in every frame and the whole song gets about 1.5 dB louder. Take one away and it gets 1.5 dB quieter.
Because the song is never decoded and encoded again, nothing is lost in the change, and it can be reversed by moving the number back. The price is the step size. The smallest change possible is 1.5 dB, which is fine for most purposes, but coarse compared with a tag. The change also travels with the file, so a USB stick, a second program or a player that has never heard of ReplayGain all play the song at the new level.
Re-encoding: the heavy way
An audio editor can normalise a song by decoding it, scaling the samples and saving it again, and that allows any amount of change, but an MP3 is a lossy format, so each new encode throws away a little more detail. For a library of tens of thousands of lossy files, that is the route to avoid, and Repair My Music does not re-encode a lossy song to change its volume.
Lossless files are different. A WAV, AIFF, or FLAC song keeps every detail of the sound, so writing it out again in the same format loses nothing that was in the file. Volume does that for those types, because they have no gain setting to change, and it keeps the original so Undo can give it back.
Loudness is not the same as peak level
Peak normalising sets the single highest sample in a song to a target. Two songs can have the same peak and sound very different in loudness, because one has a few sharp peaks and the other is loud all the way through. Loudness measures such as LUFS try to follow how loud a song sounds, which is what you actually want when you play one song after another.
Streaming services use loudness measures too, and Spotify, for example, normalises to 14 LUFS by default for its Normal setting, with 11 LUFS for Loud and 19 LUFS for Quiet, and it applies the change while the song plays. It does not alter the file.
How Repair My Music will even out your volume
The Volume tool will use the second method on MP3 and M4A files, and a lossless rewrite on WAV, AIFF, and FLAC files, and it will work like this.
- Scan. Volume measures how loud each song in your library sounds, and it does not play the songs out loud, and a large library is worked through in the background.
- Find the normal level. It works out the normal level of a song in your own library, and lists every song that sits a step or more away from it.
- Choose. Nothing changes until you press a button, and you pick the songs to move, or you pick them all.
- Move. Each chosen song is turned up or down in whole steps of 1.5 dB. In an MP3 or M4A that means changing the volume setting stored inside the file. A WAV, AIFF, or FLAC song is written out again in the same format, and its original is kept.
- Check. The program measures each song before and after, to confirm it moved by the amount planned, and a song that is already close to the loudest it can be is not turned up any further, so it does not distort.
- Undo, if you want to. One undo command puts every changed song back exactly as it was, and each song keeps its name, file size, and modified date.
How Volume compares with MP3Gain
| Compared on | MP3Gain | Repair My Music Volume Coming soon |
|---|---|---|
| Method | Changes the volume setting inside each MP3 frame, and nothing is re-encoded. | The same method for MP3. For M4A it changes the matching value inside each AAC frame, also with nothing re-encoded, and WAV, AIFF, and FLAC have no such setting, so those songs are written out again in the same lossless format. |
| Smallest step | 1.5 dB | 1.5 dB |
| Undo | Yes, using information it saves in the file's tags | Yes. One undo command puts every changed song back as it was. |
| Target level | A fixed target, 89 dB unless you change it | The normal level of your own library |
| File types | MP3 | MP3 and M4A, and WAV, AIFF, and FLAC by a lossless rewrite |
A newer free project called mp3rgain also changes M4A files without re-encoding, and Volume uses it for M4A songs.
Facts about the other tools were checked on 12 November 2026. Sources: MP3Gain, Wikipedia; Hydrogenaudio: what MP3Gain stores in a file; mp3rgain documentation.
Good to know
- Volume changes MP3 and M4A files without re-encoding, and rewrites WAV, AIFF, and FLAC files in the same lossless format, and Volume does not change WMA and OGG files, because we know of no way to change their volume without re-encoding the sound.
- The FLAC and AIFF rewrites were tried on real songs from our own library, converted into those types, on a Windows computer, and not yet on files that came from a shop.
- It levels your songs to the normal level of your own library, and it does not match an outside standard such as the 14 LUFS that Spotify uses.
- It changes the volume of the whole song, and it does not squash the music or change the difference between its loud and quiet parts.
- Close your DJ software before you make changes, so the files can be replaced.
Read about Volume, or join the waiting list.
Sources
- ReplayGain, Hydrogenaudio Knowledgebase
- ReplayGain, Wikipedia
- MP3Gain help: how lossless gain adjustment works
- Loudness normalization on Spotify