iPhone 4G ear speaker problem

Hello everyone, i just bought an iPhone 4G to repair and this is the problem :

The sound by headphone = ok/perfect

The sound by loudspeaker = ok/good

The sound by earspeaker = bad

So the defaut piece the sound was bad quality and very very low

i change the earspeaker (by an iPhone 4S one) and put a new one = no sound at all

i change again the earspeaker by an other one (still iPhone 4S) = high volume sound but horrible quality = Sizzling/crackling

I don't get it, what can be ? Thank you very much for your help