GameGrumps' Sonic 06 audio and the L4D Plane Crash sound mixed together using Audacity 2.0.3. Also works for L4D2 since you can play L4D1 Campaigns in it.
For SteamPipe users: **This addon is not converted to .VPK**, however, the normal files still work. I recommend you copy the original sound (used for backup) to the different folder (name it backup or something else) before replacing.
Audio source: [http://www.youtube.com/watch?v=OzE0LWd5SmM](https://www.youtube.com/watch?v=OzE0LWd5SmM "Click here for Audio Source!")