calculatedChaos Gaming

No, it's not a mod, no, it's not a server variable.

As per http://developer.valvesoftware.com/wiki/Impulse, impulse 202 is the 'sound spray', a custom wav file that you can play to nearby players (assuming they've opted in).
To opt in, set cl_customsounds 1 in console (this tends to reset so to make it permanent set it in autoexec.cfg).
To set your 'spray', cl_soundfile "path relative to tf/". To play your 'spray', 'impulse 202'.
Contrary to the page, the maximum size for TF2 is 512KB, just like actual sprays. In addition, the sound uses the same cooldown as normal sprays, so you can only do one or the other every X seconds.

I was messing around with another dude with this, and this is what we came up with:
http://www.youtube.com/watch?v=5mQlOulDRLE

Fun stuff.

A bit of technical info here http://www.reddit.com/r/tf2/comments/xtb0j, such as the file requirements.

Oh man. OH WOW.
I have a folder full of about 2000 sounds from Atomic Bomberman, many of which are pure gold quotes from Charlie Adler and Billy West. And almost all of them are under 512 KB.

This is going to be fun.

If the sound exceeds the limit, will the sound be cutoff, or will it just not work?

It shows you that it's too large and won't work at all.
It'll only show you on mapchange/retry.