Audio Tuning wizard: "Use headphones" is suddenly grayed

It often happens, that "Use headphones" checkmark is suddenly checked and
grayed.

The only way to make it accessible again is a fully new installation of XP.
Replacing the audio driver, directX, RTC.DLL and others did not help.

What can i modify in order to make the "Use headphones" check mark accessible?

I found out a registry key HKCU/Software/Microsoft/RTC/AEC changes from 1 to
0 and back, when setting or resetting the checkmark.
But this does not help, when it is grayed.

I don't want to ask our customers for reinstallation of the system. Any idea
of a shorter procedure (i suggest, that this problem is known by others)???!!

--
Karl-Rainer Sander
Mainz, Germany