Best way to even out the volumes of different recordings without actually changing the recording (like compressing or limiting which will make the recording sound a bit different) is to take your MP3's, and 'Normalize' them all. Some kind of mp3 editor will probably be needed (I'm a Mac man, so probably of no help).

Normalizing makes sure that the loudest peak of each recording hits 0dB. If they all hit 0db at their loudest, this should make for a pretty even comparison.