for some MBP, i know you are having problem of this. Win7 startup no sound, it's auto mute.
now from a chinese forum i found the solution, and i will post the picture of how to do that, it's chinese, but you should be recognising it.
Firstly, reboot Win7, and let it stay on auto mute.
Now open Control Panel, follow the picture.
When it's done, disable the mute, let the sound back to work.
reboot, and try it.
(sorry the pictures are not in sequence, i don't know how to do it)
now from a chinese forum i found the solution, and i will post the picture of how to do that, it's chinese, but you should be recognising it.
Firstly, reboot Win7, and let it stay on auto mute.
Now open Control Panel, follow the picture.
When it's done, disable the mute, let the sound back to work.
reboot, and try it.
(sorry the pictures are not in sequence, i don't know how to do it)