Every SUV and most cars I have ever owned and or ridden in do that when you have only rear windows down. Its due to the air rushing past
the open rear window, rather than into it, which creates a negative pressure inside the vehicle. (think about how a big rig creates a suctions behind it, which small cars use to save gas, same concept) Atmospheric pressure rushes in to equalize the pressure, once the pressure is equalized, the air still rushing past creates a negative pressure inside the vehicle... and the cycle repeats.. rolling down a front window stops it happening, since when the front windows are down air flows into the jeep, preventing the negative pressure from occurring.
As for the sound, The way speakers create sound is by vibrating a membrane, which moves air back and forth, well, think of your jeep as very large subwoofer... you get the picture...
The reason it only happens with the BACK windows is due to aerodynamics... The front windows dont have a flat area in front of them like the rear ones do (That flat area being the front window), and when you open a front window, air rushes IN the window, rather than PAST the window like in the rear...
99 WJ Laredo
2.5" Budget boost, 265/75/16 tires, K&N air filter
1992 YJ 4.0, 5 speed 6", lift 33's
1992 XJ 4.0, auto, 3" lift, 31's
1999 XJ 4.0, auto, 3" lift, 31's
1985 XJ 2.5, 5 speed, no lift, 255/70/15's