Well, all the information is on the internet, but here is my summary:

If you only stream from the internet, you won't need or maybe even be aware of ReplayGain, because all the platforms pre-process all their files to a uniform volume level, just like it's done with radio broadcasts. Normally this \"standardized volume level\" is -12dB to -14dB LUFS. You can look up LUFS for yourself, if you are unaware of 'loudness', and are interested.

Essentially, the streaming platforms do something just like ReplayGain for you, because without it, from one album to another, or from one track to another (from a different album), you would find large changes in loudness that would make you reach for the volume control all the time....

Anyone who grew up with digital music before internet streaming will know that files ripped from CDs to FLACs or bought as FLAC files, have big variations in volume from album to album. In my collection, I have up to 13dB difference between various albums - the volume level is inherent to the PCM data itself, it's not a problem with the ripping process.

Before streaming existed, people played their digital files on media players, included in all Windows, MAC, & Linux-based PCs. Most of the best media players realized very early on that having to change volume frequently was annoying, and a standard means to deal with this was published back in 2001 - ReplayGain.

ReplayGain just adds 4 tags to every music track (see pic), but it does not affect the actual music data in the file in any way. It is just a hint to any media player to adjust the volume if desired. Without ReplayGain, and the means for a given player to adjust automatically, according to those tags, you will find you have to change volume a lot if you play a sequence of albums or playlists*

The music files are examined by software to establish average gain and peak gain on a whole album (or any set of files), and also per-track, on that album. The software then adds the data values into the tag fields to signify volume changes that can be applied. It is curiously missing in Evo's, but exists in all good PC-type media players. Foobar2000 is a good media player as it not only respects RG tags it finds in files, but can also scan FLAC file to set up the right tags for your files.

Typically, media players let the user choose between no ReplayGain, AlbumGain and Track Gain. This selection would need to be added into StreamMagic's GUI.

In my collection, I have FLAC albums (ripped from CDs and bought separately) that vary by 13+dB in volume. That's a lot. It's not a big issue changing volume if you play physical CDs, one after the other, as you have a chance to fiddle with the volume dial while standing up, but if you have a media player (like Evo) that you can cue up many albums or tracks on, then to have one album or track suddenly louder or quieter is a shock!

*if you use your own playlists for your FLAC files, where you might have tracks from many different bands and albums in a list, it can be even more annoying to have sudden volume changes - think: party playlists, or late-night music. I have about 125 playlists, with some having 100 tracks in them, but sadly, it's only my Evo that cannot even out the volume for me - no problem on a PC!

ReplayGain controls and obeyance for FLACs