I highly doubt that it's the result of MP3 compression.
I think it's a combination of things: -
1. Whether the songs people are using were mastered before or after the "loudness wars" began. That can make a massive difference to the perceived volume as the waveforms of some modern dance tracks are sausaged to feck (i.e. over-compressed just to make them as loud as possible, but sometimes at the expense of sound quality).
2. The levels of both the individual tracks and the overall recording of each given section.
3. Whether the DJ mastered their section.
I would say the best way to approach this is to stipulate A) that a DJ should know how to use the gains properly and to set the recording level properly and B) that they shouldn't master their section themselves. Then, when you have all 4 sections, you would lay them out in a DAW in the way you want the overall mix to flow, then you'd adjust the level of particular mix sections if necessary to make sure that the overall perceived volume level is consistent (as best you can) from section to section, and then you can apply some light mastering if necessary, but the last thing you want to do with that is sausage the shite out of the entire mix. I think it's fine if that leads to the overall mix sounding a bit quieter than it otherwise would, as people can turn the volume up - at least the levels would be consistent throughout (or close anyway).
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