Replay Gain on Mac

It is very easy to set and use ReplayGain, which normalizes the volume automatically, for music files using foobar2000 on Windows. But it is a bit of trouble for Mac and iOS.

Here are some freeware:
1. MP3Gain Express: a very powerful tool ported from Windows version. Only for MP3 and AAC.

2. ReplayGain Scan Utility: Since many music players (including Apple's) don't support ReplayGain, another way is change the amplitude directly. So this is the tool doing that.

3. xACT: A powerful tool for converting audio formats. It has a utility that scans ReplayGain for FLAC.

4. XLD: A popular tool for ripping CD or converting formats. It only scan ReplayGain when ripping CD or converting to MP3.