what audio recording app for an iPad 9.3.5

You’re welcome. I should have included I just search for “audio recording” and that one was amongst the first few in the list, but was the only one of those few I checked that still supports iOS 9.

Have you searched the App Store? You will have to check the App’s compatibility section to find which ones still support iOS 9.

E.g. Like this one -> https://apps.apple.com/us/app/n-track-studio-9/id1130289718