You can just place an ambient generic in your map and choose the sounds you want but unless you create triggers, the sound will only play for one round if it's online!
Now seeing as it's your 'Sounds' you have to create a folder in the Half life 2 directory if you don't already have one and copy the sounds there!
You must then write the directory of the sound (location) and click ok.
When you play the map hopefully it should play although remember that if you want others to hear it, they too, must own the sounds in the exact same folder so instead of just placing them in the folder 'Sounds' make a folder within it called ambient and write a .txt saying that the players must place the sounds in this folder etc etc.