This will fix the volume on the computer. If you also did the audio enhancement described above undo it.
1. Create a folder on the root of your c:\ drive, call it "CirrusDriverFix".
2. Install 7-Zip in your Vista or W7.
3. Put the Mac OSX Install DVD that came bundled with your Mid 2009 MBP in your DVD drive then right click on the BootCamp\Drivers\Cirrus\CirrusAudioVistaxx.exe (xx=32 for 32 bit and 64 for 64-bit) file and choose "7-Zip > Extract Files..." Enter c:\CirrusDriverFix to extract the files.
4. Once the files have been extracted go to the c:\CirrusDriverFix folder and delete the original cs420x.inf file and replace it with the one attached to
http://att.macrumors.com/attachment.php?attachmentid=182216&d=1247310061 make sure the you rename it to cs420x.inf
5. Go into your device manager, right click on the Cirrus Logic under "Sound, video and game controllers" then choose Uninstall and make sure to check the box "Delete the driver software for this device."
6. Click on Start > type "cmd" then right click on the cmd.exe that shows up and click "Run as administrator".
7. type in CD "\CirrusDriverFix"
8. type in dpinst /F and then go through the install, it should "force" the install.
9. Enjoy your sound.
10. Thank ChristianZ and everyone else except Apple, Cirrus Logic, and Soft Reset
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off some random dude on the apple forums... hope this helps you out