Motherboard is the main circuit board that everything attaches too, including the Processor.
Some motherboard brand names are Asus, Gigabyte, MSI, Asrock, etc.
It should be printed on the board itself.
Right click your "my computer" icon on your desktop, properties.
Bring up the hardware tab, then device manager, then system devices will give you the Chipset, but not necessarily who makes your motherboard.
Last, onboard sound may work (although I've often had issues trying to use it)
Any decent PCI sound card will take less CPU cycles and give you better performance.