From the manual (Error opening sound device - Audacity Manual)
-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.
For an intermittent fault as you describe, the problem is very likely to be due to a lost USB connection.
Note that the connection only needs to be down for a fraction of a second to cause Audacity to loose the device. The computer will try to reconnect the device if the USB connection becomes available again, but even if successfully reconnected, the device will be allocated a different id by Windows, so Audacity will not be able to recognise it as the same device.
Common causes for lost USB connections:
- Faulty USB cable (common)
- Faulty USB connector (either end - fairly common)
- USB hubs (don’t use them for real time audio or video - usually OK for printers, keyboards, mice, other hardware that is not time critical)
- High CPU usage causing USB to be temporarily suspended (unlikely unless other apps are putting high demand on the CPU)
To avoid having to restart the computer, check the USB lead and connectors, then “Transport menu > Rescan Audio Devices”. This allows Audacity to look again for connected (and re-connected) audio devices.