This is actually pretty easy to do.

There are just a few things you need to remember:

• It must be placed within the C:Program FilesSteamsteamapps<your username>half-life 2hl2sound folder or within a sub-folder of that folder.
  (NOTE: it can be in any Source game sound folder; that's just Half-Life 2's directory)

• When browsing for it under the "Sound Name" keyvalue under an ambient_generic, you must type out the file path leading to the file starting from the sound folder within the sound name text box under the sound resource browser. For example: If I have a sound named thunder_loop.wav and it is directly under the sound folder under the hl2 folder, I would have to type in the sound name text box: "thunder_loop.wav" without the quotes of course. Hammer already knows that it should be in the sound folder, but you just tell it what sound to look for. You must add the tag at the end; otherwise it will refuse to play, so if it's an .mp3 you should type: music.mp3 as part of the sound name.
  (NOTE: If you try to search for it through the sound resource browser's filter, it normally will not show up unless you have restarted Steam, Hammer, or the SDK after placing the sound file within the sound folder. Even then, the sound may not show up or even play when you click on it, but it should play however when tested in-game.)

• Check out Orpheus's tutorial which basically says that you can have: MP3 @22050 and mono. Wave @11025 and 8 bits. Wave @22050 and 8 bits. Wave @44100 and 16 bit. as well as .mp3's. But be aware, that .mp3's will NOT loop automatically, you would have to manually make a logic_timer set to tell the sound to play (again) after the allotted time of seconds the sound actually lasts, once the sound is finished, the logic_timer will fire its output and the sound should play again as if it were looped.

-And that's all you really need to keep track of! Also, you can use these sounds with any entity that has a "sound name" keyvalue or a "start sound" keyvalue or anything with the word "sound" in it!