ok i think i can help (wha me!?!??!). What u want is so that when u enter an area u want a sound to play right? This is very easy.
Create a block. This block must be placed just before u enter the part where u want the sound to play. For example if theres a door seperating this part from the next the put the block in the door. This block must stretch to cover the width of the room before it. This is confusing so I'll make it simpler. Lets say theres a hallway before a big room right? Just before the hallway opens up into the big room put that block. Strecth the block so that it covers the WIDTH of the hallway. This is to prevent u sneaking around it and not activating the wav file. Make sure it reaches the bottom of the hallway to and a considerable distance in height to prevent u jumping over it. In case u haven't guessed, this block will be the trigger (dont worry about texturing ur not gonna see the block). Select this block and make it a trigger_once. Now in the big room place an 'ambient generic'. On the properties for it it says WAV name. If it doesn't, get the new Hammer (3.5 beta) with updated Half-Life.fgd. Select the wav file u want. Give this ambient_generic a name (For example 'bigroomsound'. Go to the properties for the trigger. Where it says target give the name of the ambient_generic, in this case 'bigroomsound'. And i think that should be it. When u compile u wont see the trigger. Walk into the spot where it is and the sound should start. If it doesn't there could be something wrong with the WAV file, i cant help u there. I hope this helps if it doesn't, well, um, be clearer next time!!!