You are right, you do not have control over the fade out time for an
audio clip in WMM. But you can if you use an audio editor like
Audacity (free Open Source). You import the first mp3 you want into
Audacity, choose where you want the fade to start and end, trim there
rest of the clip, then save the results to a .wav file. Import your
wave file into your WMM project, add it to the Audio/Music track, and
you are good to go.
As a side note, my experience with WMM is that it doesn't like the .mp3
format. It will import the audio OK, but when you try to close edit on
a beat, you can run into troubles. Whenever I use an .mp3 file, I
convert it first to a .wav, then WMM is very happy and close editing is
no problem.