This example requires ffmpeg to have been built and installed from source on your Mac, which you should have done by now at some point, but ffmpeg is merely used here to add a .wav header to the raw PCM samples, any other method of doing so will work for this.
After constructing the .wav file, it is played using the macOS-specific afplay command. The .wav file can also be played using Quicktime Player or airdropped to an iPhone for handheld playback when desired.
( printf "hello from macOS" && uname -a ) | ./fsk | ffmpeg -y -f s16le -ar 11025 -i - -acodec pcm_s16le /tmp/tmp.wav && afplay /tmp/tmp.wav
Runs arecord such that it listens to the default audio input device, outputs raw PCM signed 16-bit little-endian integers at 11025 sps, and pipes the output into defsk, which then outputs the decoded bytes to the screen, with diagnostic messages suppressed:
arecord -t raw -r 11025 -f S16_LE | ./defsk 2>/dev/null