Revisit the last transmission. We think this transmission contains a hidden message. There are also some clues clue 1, clue 2, clue 3.
RX-SSTV output for clue1.wav:
RX-SSTV output for clue2.wav:
In the message.wav seems to be another flag embedded, lets try to extract it with steghide and the given password:
$ steghide extract -sf message.wav -p hidden_stegosaurus -xf flag.txt
flag: picoCTF{the_answer_lies_hidden_in_plain_sight}