Windows can see my USB audio input but Audacity 3..5.1 doesn't

This was working one day ago but today although the USB mixer shows as the recording device the signal is not getting through to Audacity. When I click the record button in Audacity it responds with a 9999 error code although nothing has been changed in preferences. The diagnostic log shows “17:24:54: Error: Failed to load shared library ‘avformat-55.dll’ (error 126: The specified module could not be found.)”. This dll is not in the Audacity installation files or anywhere else on my PC. Will a complete reinstall fix this?

Try simple things first, Do…Tools…Reset …Configuration and then setup audio again…try different hosts

Thanks AudyMusik for your reply, Already had tried as you suggested except a different host" as I don’t have another computer available as an alternative host. Strangely Cakewalk which I also use for multitrack mixing and mastering will now also not see the incoming recording signal and Windows still shows the recording signal in the system settings. Although the original error message no longer appears when I launch Audacity, as soon as I click the record button an error message "error opening recording device, error code 9999 : unanticipated host error. I can’t trace what is causing this problem, any help appreciated.

Make sure you have the correct recording device selected in Audacity Audio setup as you see in Windows.

“different host” … I did not mean a different computer…I meant try different “host” setting in Audacity Audio Setup

Sometimes uninstalling and re-installing the driver fixes “weird problems”…

Open Windows Device Manager and Universal Serial Bus Controllers. Try to identify the USB audio device. If you aren’t sure, unplug it and plug it in again.

Right-click it and then click Uninstall Device.

Unplug it and plug it back-in. The driver should automatically re-install and it MIGHT start working.

Most of these things use the standard built-in Microsoft drivers. If your mixer has a special driver you might have to manually re-install it.

That’s NOT the problem. avformat is part of the optional FFmpeg import/export library used for importing/exporting certain formats like AAC that are not natively supported by Audacity.

Thanks again AudyMusic, I guessed you were referring to the host setting in Audacity after sending my last email. think it has something to do with the recording port driver that Audacity insists on using in the audio settings dialog box, which does not. appear in my Windows list and I can’t find a way to change it. the only options listed under the Host dropdown box are Windows Direct Sound, MME and Windows WASAPI. Below the Host dropdown box it states “Using: PortAudio V19.7.0 - devel. revision unknown” I have fixed the issue with Cakewalk by ignoring the Wave Profiler auto bit rate selection and setting everything to 24 bit 44,100Hz in both Windows and Cakewalk, but Audacity continues to spit the dummy when I click the Record icon. The audio settings are the same for both Audacity and Windows and I have tried all the available audio format options. I think I will try a reinstall and will let you know what happens.

Thankyou for your response DVDdoug, There is no requirement for a driver for my USB 5A mixer, it just uses the Windows DirectSound driver which is working well with both Cakewalk and the Windows Sound settings. I think the problem may be with Audacity using PortAudio with the Windows DirectSound, MME and Windows WASAPI Host options. These are the only options listed in the Host dropdown box. I sussed the avformat issue but Audacity now presents a Code 9999 (Unanticipated host error) when I press the Record icon. I am going to try a reinstall as these issues were introduced somehow after I had been using Audacity successfully for a couple of weeks,

You havent yet told us what you are doing or what happens if you try options.
Your PortAudio is normal and your hosts are normal, you need to try each host option.
But you need to have Audacity set to record from the device you think your are using for recording and you can monitor it in both Audacity and in windows…
How do you know your device is even working in windows… Have you watched the level meters or "listened " to it in windows.