Garry's Mod does not have a soundtrack; this rip is a Garry's Mod animation parodying the TV show Mythbusters. Jamie Hyneman and Adam Savage, as well as the narrator and The Voice Inside Your Head, star in the short via soundclips.
In the short, titled "do feces smell?", Savage begins to introduce the myth as an "email myth" (one suggested by email) when Hyneman loses patience with Savage and shoots him with his "pop gun". Hyneman then throws his gun away, and Savage quickly recovers. Soon afterward, Hyneman is crushed by a box containing the newest member of Mythbusters, The Voice Inside Your Head. Savage and Hyneman deliver (and act on) a death threat to the Voice by throwing him into an open box of bullet-proof glass and blowing him up.
The Voice is then proclaimed "BUSTED" and the video ends with Hyneman throwing the Voice (still alive) into the back of their truck while Savage comments, "I think you work fantastically".
We end on a Mythbusters-style credit roll, but with every credit reading: