This article presents a search expression to get Foobar2000 update the ReplayGain info only for files missing it.
ReplayGain is a technology used to dynamically adjust the volume of a song when it is played, in order to normalize the overall playlist volume.
This allow users to listen to the music witout reaching all the time for the volume control: songs which are too loud are automatically played lower, and songs which are too quiet are automatically made louder.
The actual music data is not modified: only the file tags are. This makes the calculation on ReplayGain info very quick.
Foobar2000 supports replaygain info calculation. Many tutorials exist on the web, so I'm not going to discuss this part.
However, here is a handy search expression to have Foobar2000 list files which do not have ReplayGain info:
Or else, for album gain:
This will display a list of tracks missing ReplayGain info.