FFmpeg is automatically installed from the repository which has non-free codecs (--enable-libfdk-aac).
gst-inspect-1.0 output on the terminal shows it detects fdkaac.
I tried to convert .flac to .m4a with GNOME Sound Converter.
My .flac files are fine (proper lossless files).
There is no difference quality between High - Very High - Insanely High. The files are checked with Spek (spectrum analyzer) from the repository. There is cut-off frequency around 17KHz
Expectation: for Insanely High setting, the audio spectrum should be over 20KHz/no cut-off