How can I increase gain on multiple mp3 files?

Many years ago I installed something that put a gain increase option on the context menu of audio files. It worked fine but I don't think it worked on more than one file at a time. And I don't know what it was.

Anywho, I want to increase the volume on all of my daily mp3 downloads since I can not get enough volume out of them. I want tp increase on all of them at once otherwise it will take all day to go through the process.

Is there a simple and painless way to do this?