Adding sounds to sectors can help set the mood of a level. Here is how you do it.
Select the sector that you want to add a sound to. Go to the item editor and double click on the value 'sound'. JED will bring up a list of the wav files that are used in JK. For this tutorial you can just select one of theirs but in the future you may want to make your own. Anyways, select one of them and you should hear it play. Double click on it and it comes up in the item editor. What you need to do now is set the volume of the sound. You can go up to a value of 1. I personally think 0.5 is loud enough but I have been told I have sensitive ears:-) You can choose what you want. In JK when you enter your sound sector you will hear the sound you chose playing at the volume you chose.
Using a cog from JK you can make sounds that will randomly play in your level. This is good for something like a spaceport when you want random engine sounds to play at random times. The cog is called 'randomambient.cog' and is a regular JK cog so there is no downloading nessacery. Add the cog to your placed cogs and then you will have this:
The first four values are the sound files(wav's) that the cog will randomly play. Numsounds is the number of sounds(max four). Min_interval is the minimum wait that the cog will wait before it plays the sounds. Range_interval is the range of the wait(ie:if the min_interval is 3 and the rnage_interval is 4 it will wait a maximum of 7). I'm sure you can work out the others(hint-just the same as the last two but for volume). Once you have that all set up like you want it you can save your level and run JK.
Things to remember
Keep your sound volume from 0-1
If you make one sector with a loud sound in it and the one next to it with no ound it will seem silly in JK. Spread your sounds out. If you make a sector with sound volume 1 make sure that the one next to it has a volume of about 0.9 so that it changes smoothly.
Don't make the sounds illogical. If you're going to be in an engine room use an engine sound-not a wind sound.