I am regretting saying this ..

On Win 7 here, the program acts as described by the original poster.
I get a wav file window also. This isn't 'normal' to a user.

Here's what happens ...

The program has always done the mixdown in high-quality audo wav format. Then it asked what format you wanted it written in and did the conversion using whatever codec was installed in the system. Now it opens a window for a wave file then asks about the conversion afterwards. It is adding a few steps in Win7. PLUS it makes a file (and leaves it) with the wav extension that is 0kb in size (but takes resources to keep the non-existent file in memory and index).
I think this is a small bug in the program for Win7 that was missed in Beta.

sorry, my fault

Make your sound your own!
.. I do not work here, but the benefits are still awesome