foo_dr_meter: A simple Dynamic Range meter based on DR estimation formula published by https://dr.loudness-war.info/
foo_truepeak: ITU-R BS.1770-5 compliant True Peak scanner.
ReplayGain is part of the core components of foobar2000, so automatically adjusting the volume depending on the loudness of the trakc or entire album is pretty much a default feature of this player. The latter two components, especially the latter one give valuable insights into the loudness and mastering quality of a recording. True Peak can calculate the Peak-to-Integrated Loudness of a recording for example the headroom between loudest part and the maximum possible loudness of the format, or it tells you the loudness range in LUFS meaning how squished or wide the dynamic range of a track is. Really nifty if you have a huge music collection and need numbers to quickly compare releases.