speakers cut out using usb microphone

I am using a Shure AMV5 microphone connected via USB which when plugged in stops the internal iMac speakers from delivering any sound. Unplugging the USB fixes the issue.

"Sound Preferences" indicate that the internal are checked for "output". However, something about the UBS/ Mike is interfering with this.

Any ideas - is this a Mac or a microphone issue? I have searched the forums and it seems to have occurred with other brands of microphones.