For those of you who have a surround sound system and haven't discovered this feature, you just need to configure your sound devices. On the "Menue" bar select "Options" and on that drop down select "Settings". On that drop down select "Sound". This opens a "Sound Settings" window. Under "Windows Sound Devices" in this window, click on the "Configure" button. This opens the ""Audio Devices Properties" window. Under the "Volume" tab go to "Speaker Settings" and click on the "Advanced" button and of course this opens another window, "Advanced Audio Properties". In the drop down at the bottom of this window, select your surround sound system and then click on "Apply". "OK" out on this and all the previous windows including the "Restart the Flight Simulator" window until you're back in the flight simulator and then restart the Flight Simulator.
Now when you fly the Ultralight and the Leer you should hear your engines running behind you if you have your speakers set up right, perferably the rear speakers set up the same distance behind you as the front speakers are in front of you. Enjoy.