I have the Samsung charging pad. The pad itself doesn't beep, but the S4 does when you place it on the pad. When charging is complete, the screen says as much but the phone doesn't beep.

 

1) Samsung Charging Pad
2) Nokia Charging pad or Stand (DT-900 & DT-910)
3) Power Qi Charging Pad

 

The energizer one doesn't beep, and I opened up the PowerQi one and clipped the speaker wire, so it no longer beeps either =P

 

Nexus Orb doesn't beep, the phone makes the same tone it always does when it's put on a charger.

 

do you need to replace the back cover to use wireless charging pad?

 

Yes, Samsung makes one as well as some aftermarket companies too.

 

Not necessarily.

Some wireless charging coils are very thin at about 0.6mm and can be used with the existing back. There is/are thread(s) about this here somewhere if you have a poke about.