You will need the ambient_generic and the trigger_multiple entities for this tutorial. Now often in your maps you need some funky sounds, sometimes you might want to have sounds continually playing, or sometimes you want sounds triggered off when you go near something.

Making a sound play by triggering it

Now firstly, place in a trigger. Now to do this, find where you want to trigger the sound in your map, and simply put in block using only the "aaa" texture. Now below we can see a machineeeee (fig 1.0). Now want I want to happen is the "horror" sound to come on when you go near it, so I need a "aaa" block around it.

(fig 1.0)

So now place in the "aaa" block (fig1.1)

(fig1.1)

Now select your "aaa" block and tie it to the entity of "trigger_multiple". Now there's a few properties in here to fill in, however you only need to worry about the following:

Target: Put in a target name, for this tut put in horror_sound1
Name: horror1_trigger
Delay before reset: 10 (this is the time it takes to reset once you have set off the trigger, higher the value the longer it will take before it can be activated again)

Now thats it for our trigger, there's no need to tick any of the "flags" in the trigger. Moving on, put in a ambient_generic where you want, ive placed it above the area where I want the sound. Now in the properties of the ambient generic we need the following:

Name: horror_sound1
Wave Name: ambience/the_horror1.wav (now since im making this tut in halflife, it searches the pak file for sounds, if your making a map for say dod, then you would browse to the sounds folder and select an item, ive had to type it in manually for halflife to where the sound is in the pak file)
Volume: 10 - well how loud it is, obvious stuff
Start volume 100, how loud it is when it starts, 100% I believe.
Start pitch 100, same as before

You can leave everything else in there, its that simple, but you dont need to change the flags, now ive ticked "medium radius" obviously this is how far away the sound can be heard. and ive ticked "start silent" otherwise it would maybe trigger when you join the map. So leave everything else.

Well thats that, should work fine, and you can do this for any triggering you might need, this is what's used for lots of maps, you dont need to have the trigger anywhere special as well, I mean some maps just trigger off an aeroplane sound when you are walking along, so they place the trigger where they know people are going to walk past/ along etc.

Having a continuous sound

If you want a sound playing constantly throughout the map, well place in an ambient generic, and check out its properties

Name: indust_1 is what I called mine
Wave name: ambience/industrial3.wav was mine
Volume 10
Start Volume 100

Now thats all you need to do, then change the flags to play everywhere. The wav will then play constantly throughout your map. If you want it to just play once every spawn or something, tick the "is not looped box"