ffmpeg can do this. Here is an easy to follow website: https://www.maketecheasier.com/normalize-music-files-with-ffmpeg/
An example command to raise the volume 50% would be:
ffmpeg -i input.flac -filter:a "volume=1.5" output.flac
25% would be:
ffmpeg -i input.flac -filter:a "volume=1.25" output.flac
I have used Audacity before to do this as well.