How to equalize MP3 peek level ?

Yes, but remember that they are already mp3-encoded which is a lossy codec. So if you want to alter them you'd have to convert them to i.e WAV, make your change, and then convert them back to MP3 so you'll get some extra quality degradation.

Nevertheless, the thing you want to do is to Normalise your sample. This can be done in most sampleprograms, even the shareware ones!
One thing though; the level of the sample will adjust to a "normal" level but eventualy some noice will also be amplified (which is a bad thing).
This noice can ofcourse be removed with various results, but thats another story.

So:
1. Convert to WAV
2. Normalise
3. Convert back to MP3

If you´re not an audiophile the most burning software supports this feature so that you don´t have to think about it, just click on normalizing or whatever it´s called. I´ve tried this and if the mp3´s are recorded with 160 or more the sound is pretty good anyway.

I know that is possible to normalize MP3 songs through conversion in WAV and then back to MP3, but I want to avoid loss of quality during resampling. Is there any possibility to transform MP3 sound level directly, without conversion through WAV?

If you need to rewrite the actual data file into a normalized sample then you'll have to convert it.
Editing a MP3 is as bad as an gfx-artist edits an JPEG; **** in = **** out.

This is the fact:
1. You have the song in MP3 format, thus it is already compressed (damage already done!)
2. The MP3 algorithm works similar to an MPEG movie, ie a stream of data and some keyframes that describes the data, so you CANT just take the fysical file and do some magic to it, the normalize function needs a pure waveform/audiodata to operate.
3. A WAV-file does not compress (lossy) the data so a conversion from MP3 to WAV doesn't do anything special to your sample (no quality loss).
4. A conversion from WAV to MP3 does some aditional compressing (this is like saving a JPEG several times, you realize the similarities now?)

You could do a normalize "live" by using some plugin for your audiodevice, but you said you want it as mp3-FILES(!) on the CD, so I don't think there is any other way to do it.

Blame it on the guy who recorded the MP3's for the first time! ;-)

The only software I´ve seen so far that can do this is rather expensive, 3-400$ at least for a basic version. I haven´t found any freeware/shareware that works as it should.

For raya70: I already know all that you said but I was just hope that some "magic" software exists. What ever, if there is no better solution for a while I will accept your comment as answer.
For uffepuff: Do you know the name of software that you mention. Maybe I can find some piracy version.

One that I´ve tried is called Cool Edit Pro (v1.2), you can download a trial version of it from : http://www.syntrillium.com/ 

Thanks Dave, that is just what I'm looking for. Maybe it internaly works on the same way as MP3->WAV->MP3 method, but at least it is one simple and cute piece of software.

.but it doesn't seem to be automatic, so you'll need to checkup every MP3 in another program first, to know how much you need to boost it.

I could have used a prog like this to, but I got about 9Gig of MP3's and I don't want to do it manualy! :-)