replaygain applies or displays Replay Gain information for audio files.
|--version||Display the version of the software.|
|-h, --help||Display a short documentation.|
|-f, --force||Recalculate Replay Gain even if the file already contains gain information.|
|-d, --dry-run||Don't actually modify any files.|
|-r REF, --reference-loudness=REF|
|Set the reference loudness to REF dB (default: 89 dB)|
|Choose the Replay Gain data format for MP3 files. The default setting should be compatible with most decent software music players, so it is generally not necessary to mess with this setting. See below for more information.|
|--no-album||Don't write any album gain information.|
|--show||Don't calculate anything, simply show Replay Gain information for the specified files. In this mode, all options other than --mp3-format are ignored.|
Proper Replay Gain support for MP3 files is a bit of a mess: on the one hand, there is the mp3gain application  which was relatively widely used (I don't know if it still is) -- it directly modifies the audio data which has the advantage that it works with pretty much any player, but it also means you have to decide ahead of time whether you want track gain or album gain. Besides, it's just not very elegant. On the other hand, there are at least two commonly used ways to store proper Replay Gain information in ID3v2 tags .
Now, in general you don't have to worry about this when using this package: by default, replaygain and collectiongain will read and write Replay Gain information in the two most commonly used formats. However, if for whatever reason you need more control over the MP3 Replay Gain information, you can use the --mp3-format option (supported by both programs) to change the behaviour. Possible choices with this switch are:
- replaygain.org (alias: fb2k) Replay Gain information is stored in ID3v2 TXXX frames. This format is specified on the replaygain.org website as the recommended format for MP3 files. Notably, this format is also used by the foobar2000 music player for Windows .
- legacy (alias: ql) Replay Gain information is stored in ID3v2.4 RVA2 frames. This format is described as "legacy" by replaygain.org; however, it is still the primary format for at least the Quod Libet music player  and possibly others. It should be noted that this format does not support volume adjustments of more than 64 dB: if the calculated gain value is smaller than -64 dB or greater than or equal to +64 dB, it is clamped to these limit values.
- default This is the default implementation used by both replaygain and collectiongain. When writing Replay Gain data, both the replaygain.org as well as the legacy format are written. As for reading, if a file contains data in both formats, both data sets are read and then compared. If they match up, that Replay Gain information is returned for the file. However, if they don't match, no Replay Gain data is returned to signal that this file does not contain valid (read: consistent) Replay Gain information.