Hi,
I wasted far too much time on CS a few years ago (student days) and I remember the gun sounds/voices/grenades being far louder than the footsteps. You'll see a pretty light show, that will obviously indicate 'sounds' but I doubt it'll be really useful, unless you're on a large map, with few players, so 'footsteps' are really the main source of noise (think 'italy', is that still played?).
Saying that, as the other guys said, a half decent VU meter would safice. You could make a bargraph, horizontally, with one one LED lit at a time (dot bar graph). The LM3915 has that ability (bar/dot). The further 'left' the left channel bargraph goes, the quieter the sound, as with the right. Both graphs going directly to the centre of the display would indicate a big sound directly in front, or behind.
Its a great idea you've got there, as hearing the footsteps is another advantage although many used to play sans sound, as it can improve your game
You get used to checking ever corner, and being overly cautious, jumpy, and quick.
One more idea, I tend to over complicate things, all my projects balloon in complexity over time, so I'll stick with my little tradition.
[wacky idea]
You *could* (probably considered hacking/cheating/exploiting) replace the wav file for foot steps with a unique waveform. Something which a circuit can detect easily and distinguish it from other sounds. This would only be for footsteps though, or a single sound (like a grenade).
If you add a transistor the the ground of all the LED's in your display, you can turn the display on/off with a single digital line, 1/0. And with a suitable detector circuit, only turn them on when this unique sound is detected. That way, your bargraph will only show footsteps noises, plus their direction, and volume/proximity. Thats probably a crap idea, because explosions produce a multitude of frequencies, hence false triggering, so maybe a 'tone decoder' or two, and the wave file will be two or three distinct tones, lasting 0.5s?. Only when all these tones are detected do you light up your display.
[/wacky idea]
There may even be a way to bypass the wav file altogether, and replace it with a script that sends a msg out the serial/USB port to a back of led's every time the 'wav' is called by HL.
Zed, harvey, audioguru has this pretty much covered I think. I just tend to rant in the wee hours.
Blueteeth