When FSX loading screen comes up it goes through the sequence and reaches 100%. I can hear the sound of the aircraft but the screens remain black and the 100% window remains in view. If I call up task manager and click on FSX I get all three screens showing but the sound has stopped.

This only happens with some aircraft, not all.

It sounds like FSX started up and loaded okay but can't completely render the graphics as maybe your settings are too high?  FSX may work okay with other aircraft or scenery but some payware aircraft/scenery take up more resources because of all of the eye-candy.  Therefore, you'll have to adjust for these types of scenarios.  Microsoft developed a solution of some sorts for situations like this by allowing you to save different fsx configurations.  Just start up FSX and, at the startup screen, click on Settings then Save....  Save your current configuration to something like "My Default".  Now go to Customize and return all of your FSX settings to the default under each tab then go back to the FSX startup and select your aircraft, location, weather, etc. and go fly. You may just want to turn off all AI and use that configuration. If it works well, you can go back and save it to something you can remember like My_Favorite_Flight.  When you're ready to go back to the other situation where FSX worked, you can go back to Settings and select Load.... and load the My_Default configuration.

 

Hope this helps.

 

Best regards,

Jim