FLAC files are either read-only or write-only.
When recording the GUI may ask for additional data to render.
Depending on zoom level, this may require seeking in the audio
data, which in case of FLAC is not possible, and should fail silently:
ARDOUR::SndFileSource::read_unlocked()
ARDOUR::AudioRegion::read_peaks()
ArdourWaveView::WaveView::process_draw_request()