metaflac --scan-replay-gain *.flac
To clarify, foobar2000 can scan and write ReplayGain tags (metadata), but it cannot adjust the gain of FLAC files directly. Any decent player should be capable of reading them and adjusting playback volume accordingly, however.
Code: [Select]man metaflac looks promising. You probably wantCode: [Select]metaflac --scan-replay-gain *.flac