We covered the fourth part of hardware hacking series where we used HackTheBox Signals for demonstration purposes. We had an audio file with .wav format which we discovered it contained SSTV encoded signals. We used special software named RX-SSTV decoder along with a virtual cable software (VB-Audio) to connect the audio file as input to the SSTV decoder and we were able to extract the underlying image file that contained the challenge flag.
Full writeup can be found here.