Ambient_Generics are very simple to use and set up once you get the hang of them. I recommend you learn more in general though if you still don't know how to set up a simple trigger that plays a sound when the player walks in to a room. Try HalfWit-2 for all sorts of crazy random tutorials about specific entities and doodads that were featured in HL2 and other Valve games. Don't be afraid to choose stuff at random. Anything will help you get a better understand of entities in general.
http://www.halfwit-2.com/?page=tutorialsBut here's some info on the entities you need for what you want to achieve:
https://developer.valvesoftware.com/wiki/Trigger_oncehttps://developer.valvesoftware.com/wik ... nt_genericThose are literally all you need to get what you want (the scream when the player enters a room). To start, place your ambient generic where you want. IF you want the scream to come from a local spot, place it where you want that spot to be and adjust the sound radius to your liking. If you want the scream to sound like it's coming from everywhere, there's a Flag you can tick for that (play everywhere). After you select your desired scream sound, name the ambient_generic "scream" or whatever you wish.
Now, wherever it is you plan on having the player enter the room, cover that with a brush with the tools/trigger texture (transparent orange with "Trigger" written on it) applied to it. Make sure the player can't bypass the trigger brush and enter the room without touching it, otherwise it won't work properly.
Tie that brush to an entity. Set the entity type to "trigger_once." In the properties of this trigger_once, you'll want to go to the
Flags tab and set it so that ONLY players trigger this entity. Now, in the
Outputs tab, you need to set up an output so that whenever this trigger is touched, it will play your scream sound. Create a new output, and set it up so that On Trigger; target entity Scream (or whatever you named your ambient generic); Play Sound. I'd load up Hammer and screen shot how it should look in the GUI but I'm a lazy prick. You should be able to discern what goes where. The output options are pretty self-explanatory.