It is unfortunate, but many receivers have poor AGC, which would otherwise even out much of the audio. Most aircraft tend to run fairly high percentages of modulation, so if the carriers are the same strength, the audio would be reasonably consistent. For that the RF levels must be high enough to activate the receiver AGC circuit.
Can you work with your feedlines and antennas to increase the raw signal strengths? That would help, then some compression followed by a limiter would even things out for the listeners:
Real audio can be pretty bad sometimes. I do a lot of flying between China and Singapore, and some ATC transmitters have a low modulation percentage, while others are higher. The Chinese tend to be low tech, and use narrow audio filters and heavy clipping on their transmitters. So one time you turn your volume up to hear ATC, then get blasted by strong aircraft on the frequency. Then you get a hand off to another facility where the engineers set the percentage pretty high, but clip the hell out of the audio...
So if you do a bit of processing on the receive side and signals are still not totally consistent, you're not doing bad - radio is tough to tame sometimes. As long as you don't overdrive your soundcard, things should work out alright. Good luck!