You may need to open the laptop again and check the cable that connects the speakers to the motherboard if it's not damaged or not. Unplug its cable then put it back in then test weather if the sound will be better or not. Also, test the sound of your laptop using an external speaker or a headphone and see if the same problem will persist. About the USB drivers do uninstall/reinstall them.
- Go to Device Manager and uninstall the USB controller drivers.
- Once done, reboot the laptop and it should reinstall the driver automatically.
- If it will not work proceed with running System Restore.
- Here's how to run System Restore: http/windows.microsoft.com/en-PH/windows-8/restore-refresh-reset-pc
- And if System Restore will also not work do reinstall Windows.