Increasing levels

Is there any way that I can increase the sound levels on mp3 files that i have recorded with Audacity? A friend made some voice recordings for a new website, but some of the files are somewhat quieter than the others, is there some way to “turn up” their volume?

Thanks, I appreciate any feedback.

Using “Amplify” (on the “Effects” drop down menu), with the default settings will make a track as loud as possible.

Using a lower (but positive) number than the default value in “Amplify” will make the sound louder, but not maximum-loud.

[using a negative number in “Amplify” makes the sound quieter rather than louder ]