With the help of Apple Support I was able to solve this problem.
Since v10.4 a whole lot of DV camera’s are no longer put one the compatibility list causing this problem of “no sound import”. This has the same effect on iMovie!
Fortunately I still had an previous version of FCPX (v. 10.3.4) on my TimeMachine that I installed… and now everything works again like a charm.
I made a complaint to Apple to reinstall these compatibilities… but don’t know if it will help.