Lets step back a bit, please explain what you are actually trying to do?
The "Select Sound Type" menu option in the MST will be greyed out when you have opened up the CV template for a driving only decoder.
So, in this case, if you want to program the XL2 PluG decoder you can get the software to detect it and select the correct template.
However, if you want to change the CVs in the S sound module you will need to manually select the CV template.
Now as you are talking about the "Sound Type", this functionality in the software just associates the sound data sheet to the current template and it may change the labels on some of the fields. Id does not however select the sound within the sound decoder. If you wish to change that then you need to upload sounds to it.