Warning message if audio input has no audio data
I had the recorder configured with the wrong input channels in the JSON file, and so it was recording full-filesize WAV files (30 minutes long) but with no actual audio data when I opened them in Audacity. Then the WAV files would be transcoded to MP3 files of a few KBs.
So a warning message in the console would help with realizing what's up.
Related issue would VU meters (#47 (closed)) since that involves monitoring audio levels.