OK, a few things to note.
The motherboard audio chipset (ie: often referred to as onboard audio) in this case, is a Realtek chipset. The drivers can be found here: Motherboards - M5A99X EVO R2.0 - ASUS
The AMD 270x gpu uses a dedicated audio chipset to supply audio over the HDMI output. The drivers are part of the video driver package. You can get the latest drivers here: Download Drivers
Any USB audio device uses it's own drivers. Those drivers are available from the device manufacturers website.
You have to select which chipset is used as the default output device. Which device you select, depends on what device you are using for audio. For example, a device such as a TV connected by HDMI, you would select the AMD HD (HDMI) device. For speakers connected to the motherboard audio ports, you would select the Realtek chipset. For a USB device, you would select the USB device.
You can have multiple audio chispet installed and they should not interfere with each other (the exception being multiple devices which use the same chipset brand).
Having said that, "scrathiness" is typically a sign of a driver issue or less likely, bad cabling or a faulty device (ie: speakers or headset). Ensure all of your drivers are up to date. MANUALLY search and install the drivers, do NOT rely on Windows or Windows Update to search for new drivers. Windows only checks the Microsoft site for the latest Microsoft certified drivers, which are typically several versions old.