I'm going out on a limb here when I say both previous responses were right and both were wrong.
>No. It isn't backward compatible.
That's right; but the changes made by MP3 Gain are made to the mp3 file itself and the change in volume WILL be recognized and played by ANY mp3 player.
>MJ8 analyses the replay gain using the same process and then codes the result (a number it appears) in the MP3 tag but otherwise it doesn't change the MP3.
Almost right; the analysis results made by MJ are stored in a separate database, not in the mp3 file itself. If you lose the database, you lose the analysis results and would have to analyze over to get them again. Also, if you change the name of a file, you would lose the link between it and the database. The fact that MJ does not change the mp3 file could be both an advantage and a disadvantage.
You may have guessed by now, I use MP3 Gain, Radio Gain, and love it. All my files have similar sounding volume no matter where they are played or with what player.
So, MJ v7.2 is NOT Replay Gain compatible, but it IS MP3 Gain compatible.