OK Thank you so far for the help. I just recorded a sample and attached to the post.
This is AAC in an MP4 container. The ".mp3" file extension is wrong.
Edit: If you rename it to Sample.mp4 or Sample.m4a, it plays as it should. No codec issues here - what fb2k needs, is the right filetype. Peter has chosen not to allow for wrong file extensions.
VLC on the other hand, has chosen to ignore file extensions. Try to rename it to "Sample.txt" and open it in VLC. It will play.
If you press Ctrl-j while playing, VLC will tell you the codec.
ffmpeg returns this:
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: isommp42
creation_time : 2019-04-08T20:54:13.000000Z
com.android.version: 7.1.1
Duration: 00:00:10.69, start: 0.000000, bitrate: 100 kb/s
Stream #0:0(eng): Audio: aac (LC) (mp4a / 0x6134706D), 16000 Hz, mono, fltp, 96 kb/s (default)
Metadata:
creation_time : 2019-04-08T20:54:13.000000Z
handler_name : SoundHandle