For all those having problems with a -9999 error message when you try to record, for me it was as easy as unplugging the Behringer interface’s USB and plugging it into a different USB port. That fixed the problem, but it went back into mono and I had to go back into Windows 10 Sound advanced settings and make it 2-channel again.
From the manual: (https://manual.audacityteam.org/man/error_opening_sound_device.html#codes)
9999 Unanticipated host error: this means means “something is wrong”. And appears to most often be due to either:
- A lost connection to a USB audio device.
- Recent Windows / Mac update requiring that user gives permission for Audacity to access the microphone.
In the case of (1) a lost connection to a USB device, it is usually sufficient to “Rescan audio devices”
If that doesn’t work, check the USB lead and connectors.
If they look OK, unplug the device, wait a few moments, then plug it back in again and wait for Windows to find the device, then either restart Audacity or “Rescan audio devices”.
If there is still a problem, then as suggested by mdevich, try plugging into a different USB port.