MP3 downloads for audio files is highly requested, but bring some difficulties for our developers.

First, and most importantly, YouTube does not provide MP3 files which could be directly downloaded. Therefore the only thing we could do on NewPipe's side is to convert/transcode M4A or WebM streams.
The reason why this features has not been implemented yet, is quite simple:
This process requires an external library like FFmpeg which would significantly increase NewPipe's size.

Nevertheless our team is working on a solution to make this features as soon as possible available to you. The plan is to move the conversion to an external and thus optional app called Video Transcoder which you can download from F-Droid too.

Help and contributions are always welcome. If you want to take a look at this issue, please read this page on GitHub.

NewPipe uses the Android Storage Access Framework (SAF) and a the old but gold Java Input Output (Java IO).
SAF requires Android KitKat or later. However it is not supported on all devices. If this applies to your device, please disable SAF. To do this go to the Download section in the Settings and uncheck Use SAF.

Maybe you installed the F-Droid version and want to update with an APK from GitHub or vice versa? Android does not allow app upgrades when the signing keys differ. This ensures the original developer has condoned the app.

This means, you need to either get the APK from the same source, or backup your NewPipe database, uninstall and reinstall the new app. Without a backup, current app data is lost, including subscriptions, history, playlists and settings. Follow this tutorial to do so.

The final solution to this problem is to have reproducible builds for NewPipe in F-Droid. We are working on this with the F-Droid developers here.

No. NewPipe does not rely on the YouTube API, as using it requires an API key bringing with it specific regulations and restrictions. YouTube would then have more options of tracking your YouTube usage at their disposal. Therefore, NewPipe Extractor gets the information by parsing YouTube websites.