Audacity won't amplify

[AlanHK]

I've got a video sound track which is much too quiet. I just want to make it louder. I loaded the MP3 (demuxed from the AVI) into Audacity and selected the "Amplify" effect. I crank it up to 20dB increase, the preview sounds fine. But the "okay" button is greyed out. If I select the "allow clipping" checkbox, the okay comes alive. But now the preview sound is much lower, even at maximum. I expect this has something to do with some very loud sounds (explosions) later in the soundtrack. Similarly, if I use the "Envelope" tool, it won't let me drag it enough to make a difference.

How do I tell it to just amplify regardless?
If not in Audacity, in some other free tool?

[AlanHK]

Replying to myself -- I used Mp3Gain, which does what it sounds like.

I got some errors when I transcoded that to MP2, but these were at some gunshots, so clipping or distortion isn't a problem.