Notes:

See Also Ghost From True Capitalist Radio

Boy, I wonder if anyone's even gonna get this joke.
I better explain it so it can be funny!

People would call in to Ghost's radio show and troll him with all sorts of s--- sometimes causing him to rage SO hard the sound of an ocean of aluminum beer cans crashing could be heard, either from him wading through it or throwing his microphone into it. This became an entire thing, "Cans.wav".
(Wikia page on cans.wav)

The joke here is replacing Ghost and his cans with Reaper and his guns, both represented by skull faces.
The joke has been explained.
Proceed to find it humorous.

Hope this is a good video of it, can't listen to it at work: