Having once jumpseated over Africa I think the clarity of even the poorest quality transmissions in this country are perfect in comparison.
With regard to the transmissions by stations I presume you are aware of the slight off-setting of frequency for most ATC ground transmitters? If not (please don't let me teach you to suck eggs on this one) then tuning approx 5kHz either side of a ground station will improve quality no end. Of course if you do that aeronautical mobile stations will sound even more off frequency.
What you tend to find too is that after using radio in its many forms for a considerable period of time one acquires "RT Hearing" in that even the most unintelligible transmission gets decoded into something useful somewhere in the deep grey matter.
Much is to do with the quality of the microphone too of course - both in the air in your typical Cessna and on the ground. When LACC opened there were countless problems with clarity and volume of transmissions all of which were traced to the headset equipment.