A 3d sound is a sound which you can give a position in the 3d environment. The game will calculate how it should be positioned in your speakers relative to where your camera is positioned, and mute the sound if you're too far away from the camera if that option is checked for your chosen sound.
Here's the simplest way to use a 3d sound.
Open the Sound Editor
Find the sound you want to use. Right click it and 'Use As Sound'.
In the window to the right, double click the sound and make sure the 3D sound box is checked. Some sounds cannot be used as 3D sound for a reason I do not know. Changing the channel used helps for some sounds. You might also want to change some of the other options. Experimentation is the key.
You can now use the sound with the following triggers:
Trigger:
Actions
Sound - Play ThunderBoltMissileDeath <gen> at 100.00% volume, attached to (Last created unit)
Sound - Play ThunderBoltMissileDeath <gen> at 100.00% volume, located at (Center of (Playable map area)) with Z offset 0.00
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.