I got Left 4 Dead for Christmas, and it's awesome. Unfortunately, you pretty much absolutely need a mic to play. Typing or even using the in-game voice commands is way too slow/generic. I don't really like the mic on my laptop, so I decided to try hooking up my bluetooth headset to my computer (via a bluetooth dongle I have). So got that all squared away, and discovered I was required to set the handset as the default device before starting the game in order to get sound to come out (I still haven't found a graceful way to replicate output to the bluetooth AND the realtek audio). This is annoying, so I did a little research and set up a batchfile to automatically handle switching the device for me before starting the game.

This setting is stored in the Windows Registry, in HKEY_CURRENT_USER\Software\Microsoft\Multimedia\Sound Mapper, under the Playback value. You can use the reg.exe command to change this from a batch file, as below:

@ECHO OFF
echo Switching to bluetooth handset...
reg ADD "HKCU\Software\Microsoft\Multimedia\Sound Mapper" /v Playback /t REG_SZ /d "Bluetooth Hands-free Audio" /f

echo Starting l4d...
start steam.exe -applaunch 500

echo Hit enter to reset the default audio device...
pause
reg ADD "HKCU\Software\Microsoft\Multimedia\Sound Mapper" /v Playback /t REG_SZ /d "Realtek HD Audio output" /f