If you feel up to it, you can even order the part from us, grab the repair manual, and the torx screwdriver you need to open it up. But of course, we can't guarantee the sound board is the problem, not without us doing the repairs here.
If the unit will turn on occasionally, then the odds are it isn't the Sound / AC board. This board is most normally associated with the laptop getting no power. If you are having intermitten startup, then the cause is most likely your processor or logic board.