ReplayGain is nice - but note it doesn’t ’fix the compression’. Compression and dynamic range is about loud/quiet _within_ the track. ReplayGain just turns the volume up and down for the entire track, the point being so all your tracks play back at about the same level. It saves a preset on the volume knob for you essentially.

If you remember making a playlist where one song is suddenly much louder than the last, and you’re riding the volume knob on every other song, you’ll see why this is nice!

Yeah, you run an analyzer on your library and it creates MP3 or Ogg Tags that the player. Often you can leave the rest of the file as it was originally, just the new metadata tags.

On Windows I've always liked Foobar 2000 for its strong ReplayGain support, both automating the analyzing pass and respecting the metadata in the tags once saved. On Unix I was using Banshee for playback and automating analyzing with a pair of CLI tools I've forgotten the names of, one was MP3 specific and the other Ogg/FLAC-specific, as I recall.