AFTER recent Windows 10 Update (1803) > Input Device Error/Undefined external error.

Thank you sir, all these months later, I had figured this out myself last year but after moving(after a house fire) I couldn't for the life of me get Cool Edit Pro to record! (Audition is Cool edit)
Thank you!

Kevin

My Windows system just updated to 1803, and it quite hosed things up in Audition. My Focusrite Scarlett only showed up as an unrecognized device, and headphone playback was horribly overdriven.

To fix things, I had to go to Device Manager and completely remove the Focusrite device and drivers, unplug it, reboot, download and install the latest Focusrite USB drivers (4.36.5-612), reboot again, and then plug it back in. That finally got it working again.

I fixed it, its recording again!

I had the same issue after the win10 update. At first I thought it was the sound card. I updated the driver-nothing. I reinstalled Audition thinking it was a corrupt file-again nothing. There was an online post about sndvol32 being missing and was causing this- I checked and found that's not it. I also tried the suggestion about checking/unchecking the "allow control by apps" box, and that didn't work.

The fix: 1-The windows update caused registry entries to conflict. I ran a registry cleaner and defragmenter, and that fixed it.

             2-Go to Privacy settings and allow microphone to be accessed. I do not use a mike that plugs into the sound card, but this setting also seems to control the line-in input next to it. ( I'm using the Reaktek HD audio soundcard)

I had exactly the same issue today after Windows 10 updated to v1803, and I'm running the same version of Audition as you. I found the solution here: UMIK-1 not working under Windows 10 64 bit with REW. | AV NIRVANA 

TL;DR: Go to Settings > Privacy > Microphone, and try flipping the toggle under "Allow apps to access your microphone" to On.

What audio interface are you using with Audition 1.5? It seems you are not alone if you search for audio problems with the latest Win 10 update if you look on the internet.  Unfortunately it sounds as if Microsoft have dispensed with an earlier generation of working audio drivers. I think that other people are having trouble with audio in 1803 with all sorts of software I'm afraid.

I would check all the settings in Audition's Audio Hardware setup. Change them around a bit even if you go back to the original settings eventually. With any luck it kickstart Windows audio drivers to work again. otherwise you will have to try and find a way to re-install your previous audio drivers.