Hello Tino. Sorry for late reply but I have been busy and offline. Regarding your questions:
Beta version access https://play.google.com/store/apps/details?id=org.ostteam.audiorecorder.android
Regarding mp3 recording, according to the developer and documentation regarding encoding/decoding https://developer.android.com/guide/topics/media/media-formats there was some issue so we dropped it.
And about opus vs AMR (in terms of file size relative to the quality) we have not make a comparison because there was an apparent royalty issue with AMR.
Great to see there is a first version of Built-in audio recording!!
Again sorry for this late reply,