I have both the PMDG 747 and MD-11 sound sets. I purchased the PW set but use it for all the engine variants. My problem is that I'm not getting much if any sound from exterior views. If I go to the actual sound folder and play one of the engine reverse sound files it is loud and clear but in FSX when I reverse the engines on landing I hear nothing. Other sounds seem to work ok, but most of the external sounds are very quiet or non-existent. I've looked in the sound.pw folder and the sound.cfg file is there as are the files that it references. 

I purchased the FSX IAE 2500 HD sound set from TSS and was very unhappy with it using the Wilco Airbuses for the same reason. I since then stop buying their sound sets. because most aren't compatible with pay ware aircraft.

The sounds are there and they sound great. I can double click on the thrust reverse audio file in the sound directory and it sounds awesome, it just doesn't play when I activate thrust reverse in the aircraft. :(

Did you correctly set the setting in the PMDG MD-11 sound menu?

It's in the drop down menu in FSX.  Make sure you check the box that says Play internal sounds in external view.  They tell you to check this box in the PMDG instructions in order to hear all the sounds.

Sounds strange - pun intended. I have very good experience with TSS sounds for the Beechcraft King Air and DC-3 Dakota, so I would be surprised if something wasn't working right...

Yes, I have the MD-11 set to play sounds in external views. I am not displeased, don't get me wrong. I have the DC-3 soundset too and overall I love them. It's just that the outside sounds and specifically thrust reversers are missing in the MD-11 and 747. As I said, the files are there, and when I play them they sound great, they just aren't playing when I activate the reversers.