Samsung's Voice Recorder is reliable on my phone, also in the background and under lock screen. Further troubleshooting is up to you.
There's good advice by @AntS in the Smartphone App Troubleshooting Tip Thread.
* Always backup your stuff before troubleshooting.
Some Android versions require some apps to be set as Not optimised in the battery settings in order for the app to work in the background and under lock screen.
Long-press the particular app's icon on the start screen and select App info in the pop-up menu for easier access to the app's settings.
Regarding the workflow for your project I suggest focus on the end result.
Is it for personal records, for a website/blog, to be printed on paper or ..?
A Notes app could be useful, maybe, but at the risk of growing into some unmanageable Gigabytes over time when it has to hold hundreds of photos and voice recordings.
It's important that it backs up somewhere and can be accessed outside the phone - both Microsoft OneNote and Ǵoogle Keep sync notes to the cloud and can be accessed in a web browser on a PC.
I hope you get it sorted out.
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Hans
