Try
this for some more details on custom sounds using the
ambient generic entity.
While the entity guide is dated, It's still useful for general Info. Search the forums for ambient_generic/custom sounds/pak/etc., for more detailed info.
A couple of notes:
Some HL sounds have cue points, and some don't. Ones with will loop continously 'till triggered off, and ones without will only play once until triggered again, regardless of flags/settings.
If you edit cue points on HL sounds--I use Goldwave--you'll need to include the edited .wavs with your map distribution. Hopefully the sound you need has cue points, so you don't have to mess with it.
I personally--while using a pak explorer--copy all the original HL sounds into my Hammer/sound folder. This way, I can check them out without another piece of software. 'Do the same thing with models and sprites too.
If you can't get it to work, check out someones map that has sound, and see how they did it. Decompile if you have too, but many maps here come with the .rmf file so you can edit/look.