Having one or two mp3 files whose volumes are either too loud or too low? Try MP3 Louder.
It’s a free online tool that offers a quick fix to the problem. No registration.
All you have to do is simply add the mp3 file, select the appropriate task (increase or decrease volume), select the desired decibels and channels, and hit the Upload button.
It may take a while, depending on the mp3 size that you upload and your internet connection quality. In the meantime, you can do something else while waiting until it’s done.
When it is finished, you’ll see a download button appearing right on the same page.
If the result doesn’t meet your expectation, or if you simply need another solution without having to install another software, there is mp3trim. It’s a free portable mp3 tool made for Windows.