This is something that drives me crazy being an Acoustic engineer.
Assuming decent speaker distribution...which most AC
have, it is VERY easy to level off the sound with a simple electronic device but most systems don't seem to have one, or have it set correctly.
Mic positioning plays a huge part as was previously mentioned. Many times the mics are poorly positioned relative to the source and just as much background noise is amplified as from the source. (We can address this with noise cancelling mics which are generally inexpensive also)
Systems can also be automatically compensated (up/down) for interior noise in the pax cabin during taxi,takeoff and cruise.
Then again, if bad sound were leathel Audio would be the leading cause of death.