No buzzing if the audio cable is disconnected.
No buzzing if the Interface's USB cable is disconnected. Yes buzzing if the interface's power is disconnected, but the USB is still connected.
Yes buzzing, no matter which output of my interface the speaker is connected to (there's output 1,2,3,4 on the back). I'm not sure what other source I could try. I mean, I plugged a dynamic mic into it, no buzzing, but in that case it's not connected to the graphics card any more, anyway.
It's a Focusrite 6i6 USB interface. I've tried every USB port on my computer, back and top.