Hi all ! Helped by this thread, I found a free solution, using the free trial version of iMazing and the last version of the app iTalk 4.7.5.ipa file I've uploaded on my Dropbox. It consists in manually reinstall iTalk on your device and then backup your files via iTunes or iTalk Sync, rather than using the paying version of iMazing to get all your files back.
It will work only if the app worked before (I have iOS 13.2.3 but it may works on iOS14, not sure about iOS15...), and you will be able to recover your data only if you still have the app on your home screen (uninstalled), and so logically, still have the app data. If iTalk is no more on your home screen (totally uninstalled), you may already have lost your data... In my case, iOS automatically uninstall apps to save disk space, but fortunately it doesn't erase app content.
1) download iMazing, install it and click on "continue trial"
2) in iMazing, select your iOS device (or a backup ? ~User/Library/Application Support/MobileSync/Backup/)
3) click "Apps" section, then find iTalk. Right-click and "restore app"
4) Select iTalk 4.7.5.ipa file I've uploaded on my dropbox. IMPORTANT : it won't erase your audio files, it only will reinstall the app as you could have done from the App store or by clicking the app icon
5) Click on "select" and tadaaa, iTalk is back with the free version of iMazing.
6) Backup all your iTalk files via iTunes app files manager or "iTalk Sync" which is also in my dropbox link (Griffin's link is dead).
Hope it will prevent some people here from losing data ! It can be huge losses sometime...
PS : Griffin (iTalk's developer) was bought by Vinci and since 2017 they stopped to give a **** about iTalk, even if it still work on most of versions of iOS today. Take care of your data, because they don't.