1. Right-click Dirt 3 in your game list, and click on Properties
   
2. Click the Local files tab and select Browse local files
   
3. Inside the Dirt 3 folder, go to audio\speech\en\
   
4. Find the en_generic.nfs file and delete it

This will make the narrators shut up. You'll find that the game is pretty enjoyable once you can actually drive around without being interrupted every minute. Your co-driver, who is actually helpful, will still speak to you during the game, since his lines are packed into different files.

If you're playing Dirt 3 in a language other than English, then I imagine you could delete the announcers' voices as well, if instead of the en_generic.nfs file in the en folder you do the same for the folder and file labeled with your language - fr, ge, it or sp.

That's it! Enjoy the game.

If you found this guide useful, consider buying me a can of coke on donatty[donatty.com]
or boosty[boosty.to]
Cheers!