Hi Timothy. It's a shame I didn't notice it right from the start but the problem is that the sound of your Mosquito is aliased to an aircraft you don't have. It worked for me as I already had the Corsair it's aliased to installed to my own FS9. I suspect this Mossie started life in FS2002 or CFS2 which would explain that as the Corsair is a default aircraft in both. Rather than confuse you by explaining what Aliasing* is all about at this stage you can fix this by editing its Sound.cfg as I explained previously. This should only take a few moments but please do it carefully using the correct name.
[fltsim]
alias=c182\sound
Alternatively, copy the Sound folder of the Comet & paste it into the Mossie overwriting files/folders when prompted. Either method should fix it & the Mossie should now display on the selection menu under DeHavilland. You can change this afterwards if you wish.
PS.
The DeHavilland Dash which does not work.
The default Dash 8 is one of the unflyable AI Only drones used as AI traffic. The others are the MD-80 & Piper PA-28 (I think as I'm not on my home PC). These are not not supposed to be listed on the menu & you're finding problems that don't exist.
*PPS. To find out about Aliasing click the link to Grumpy's Lair in my sig below. You will find many helpul tips & tutes that save so much time explaining all this stuff. Also, please take a few minutes to read my Windows Basics & Working with Windows articles. Hopefully this should make it all much easier to understand.