You can import mp3's into audacity apply the amplify effect and set the peak to the same value on all files and then export as mp3 again.
To import/export mp3 files in audacity you need to have Lame installed.
Also you need to be aware of 2 things:
1) mp3 is a lossy format, so everytime you compress some audio to mp3 you're "damaging" it, if you import an mp3 into audacity it's decoded into raw audio and then when you export back to mp3 you're compressing again and that means adding again more damage to it. Everytime you decompress/recompress audio from/to mp3 you'll be adding extra damage to it. There some apps that can work directly on mp3 without decompressing/recompressing it and do some operations to them such as splitting/cutting but I don't know of any that can do amplification.
2) amplifying all the files to the same peak value doesn't mean that them all will sound as loud as each other, that will depend a lot on the dynamics of the song. If you have a very loud peak at some point in the track but most of it is quieter than the peak, then it will sound quieter than another track which most of it is near the peak. In this case you might need to use dynamic compression in order to make them all sound "as loud" as each other.
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