I would perform an F9 ASUS Factory Restore. Press F9 at the ASUS splash screen and choose to restore Windows to the 1st partition. This will overwrite all data on your OS partition, so backup any important files. It will reboot the system several times, so be patient.

If the speaker problem persists, return your laptop to the retailer for an exchange to a new G73. Since you report you just got the laptop, I'm assuming you are still within the retailer's exchange/return period.

If you'd prefer not to restore Windows and retain your current installation, I recommend booting into a Linux "LiveCD" distribution and attempting to reproduce the problem. If the subwoofer still is the only speaker to produce audio, you've confirmed it's a hardware problem.