Welcome my good friends! :)
So you have a sound, and it won't play in css for some unknown reason? Then look no further!
If you know a little about sound for the source engine, you'll know that the sound has to be in the .wav format (*unless it is a piece of music, in which case, an mp3 format): yeah it's preeeetty obvious to us :)
...But! What isn't so obvious, is that the .wav format has a sort of format type within it! so do all audio files in fact. These special 'format within a format' are not uncommon, in fact, every audio file you have come across is saved as a 'format within a format'.
Basically, Css sounds have to be saved within these .wav file parametres:
Wave PCM (Signed or unsigned)
8 Bit or 16 Bit
Mono or Stereo
44,100 Hertz or Less
To save the .wav (or any other sound type) with these preferences:
Locate and open the sound
In the top (File Edit Effect etc) go to Effect > Resample
A box like this should open up
If your sound is higher than 44,100 Hertz, then Lower it to 44,100 Hertz or less (a lower number will result in a less 'detailed' sound (think of an image's resoloution - higher = better quality, same scenario with the fidelity/amount of Hertz)
If your sound is less than 44,100 Hertz (16,000 Hertz, 22,050 Hertz and 32,000 Hertz are the most common rates of Hertz after 44,100 Hertz) then don't change it to 44,100 or any higher than the original sound, unless you want to waste space: why? Because the wav is being allocated more space, but it doesn't affect the quality, so basically it's wasting space.
At the top go to File > Save as
Now the settings should be so: File name: This is the name of the file.
Save as type: This should always be 'Wave (*.wav)'
Attributes: Here's what I was talking about earlier. Scroll up to the top and choose one of these formats (The list is ordered from the worst sounding to the best sounding) - PCM Unsigned 8 Bit Mono - PCM Unsigned 8 Bit Stereo - PCM Signed 16 Bit Mono - PCM Signed 16 Bit Stereo
Anything other than these formats will not be played by Counter-Strike Source. This is the case with all source games