You have to provide more concrete information:

have you done any investigation on the headset: you said you have replaced the mainboard just because earpiece doesnt work..

-earpiece model

-tested in another known-to-work phone?

-tried n9qttweaks to set volume higher or alsamixer? http://everythingn9.com/increase-the...your-nokia-n9/

-any other earpiece works on n9?

-how about normal headsets, without mic?

-is the problem only during calls or also during music playback?

-Tried to reflash firmware to correct any issues in previous?

n9 earpiece is compatible with older nokia phones like N95. Try your earpiece in these older phones.
But it is not compatible with iphone type headsets. try to see if it works on phones with iphone type headsets.(if it works, its obviously not an n9 headset).

Finally, you can always use bluetooth headsets(assuming it works). They have more functionality than normal wired headsets.