The sound editor can be opened by typing "editSounds" in the console. It will probably take a while to load, because it is scanning and loading a bunch of sounds, be patient. The sound editor was designed for a sound guy (non-level designer) to run around the levels and create a spooky aureal experience, so it works independant of radiant.

When the sound editor is up, you can switch to the game easily by clicking "Switch to Game" but when the game is up, you can't exactly mouse over to the sound editor. There are a few ways of switching back, but the easiest is to bind a key to "editSounds" (type "bind <key> editSounds" in the console).

At first glance, the sound editor is kind of daunting, but it's really not all that complex when you get down to it. The bare minimum you need to do to make a sound play in the world is click the 'Drop Speaker...' button (near the bottom). This will place a speaker in front of you. Next browse the tree in the middle and double click on a sound (I'm fond of loop_fan_1 in the sound/fans.sndshd file). Technically that's all you need to get a sound emitter in the game, but there are obviously other things you can do.

To move sounds around in the world, use the y+ y- x+ x- z+ z- buttons in the top-right of the window. It may seem like a clunky interface, but it actually starts to feel right after using it for a bit (you can just click on a sound, move it left a few units and go on your way).

A lot of the options present in the window can also be changed in the sound shader. If the value is set to 0 in the editor, then it uses the value in the sound shader. This means if a sound shader has shakes, then setting "shakes" to 0 in the editor will not disable the shakes. This also means if a flag is turned on in the sound shader, it cannot be turned off in the editor. Keep this in mind if a sound is looping even though "Looping" is not checked.

Here's a brief description of all the check boxes you can check:

The "Trigger" button will trigger the selected entity. Useful for testing the sound on an entity that needs to be triggered first.

The group name, and groups selection box lets you group multiple sounds together for ease in searching and selection (in theory) I don't think anyone actually ended up using that feature though, which makes me think it doesn't work right.