ReplayGain

[simbun]

So it's clear that at this point you ARE modifying the actual MP3 audio data itself as if you'd just performed an Analysis step you wouldn't have had the MP3GAIN_UNDO tag (as there would be nothing to undo), but it's a losslessly reversible process so that's fine.

MP3Gain will be using the original ReplayGain algorithm whereas foobar by default now uses the newer EBU R128 implementation, which will account for some of the differences.

When you calculate ReplayGain in foobar (as long as you don't 'Apply gain to file content') you'll just be adding ID3 tags which is why they're picked up by ffmpeg.

At this point it's really down to you. If all your players support the ReplayGain tags then this is the best approach as you can calculate both track and album gain and flip between the two if desired, but if you have some players that don't support the tags then you'll need to use MP3Gain as foobar doesn't generate undo tags so 'Apply gain to file content' is a destructive process.