In the real world you can't hear the boarding in the cockpit because the avoinics cooling fans are much to loud and to be honest people entering an airplane just do not make much noise. Even with the cockpit door completly open you don't hear it. In FS2Crew it is very loud though.
Is there any way how to remove the boardingsounds without having to turn the FS2Crew volume settings down completly (I'd still like to hear the ramp agent bringing the loadsheet, etc.).

 

I would simply edit them myself, but since FS2Crew does not save soundfiles in the common formats I have no way of editing them.

Try turning the environment volume or cockpit down during boarding .