Guano,
1) I dont know how old your computer is. But some of the older computers when we switched to AH3 offset the sound 30 to 60 degrees. As an example, I would be looking in one direction where I hear the sound but see the tank left or right of my position 30-60 degrees from where I was looking. This may or may not be the case for you. It was for me, and before I upgraded computers I did upgrade my sound card which rectified this situation.
2) Again it would help to know the age of your computer. If you have your setting turned down so that you can run the game with an older video card, you will not be able to see the Enemy tanks at long distance. Otherwise, it could be guys in Panthers, Tigers etc that have better zoom capabilities than the standard M4s, Panzers, or T-34s etc.
3) The M4-75 is the tank that carries Rockets. You can select Rockets or alternate ordnance by using the "Backspace Button".
It helps if you are inside the tank looking at the Gun sight. It will tell you weather you have selected HE, SMK, AP, or Rockets on the bottom of the reticle.