I am unsure if this is your problem, but I have encountered this scenario before.
I bought an Asus G60 and as soon as I started up the laptop no sound issued forth except the bios initiating. I tried everything from changing drivers and settings to searching forums online. What I discovered was the system was stuck in headphone mode because of a faulty headphone jack not releasing pins telling the motherboard the headphone were pulled out.
If headphones or external speakers work but no sound out of the laptop speakers, this may very well be your problem.