import os
os.system('cat p.wav')

python first.py |  ffmpeg -y -f wav -i pipe:0 -f mp3 - >hi.mp3

import sys
with open('p.wav', 'rb') as content_file:
    content = content_file.read()
sys.stdout.write(content)

python first.py | ffmpeg -y -f wav -i pipe:0 -f mp3 - >hi.mp3

pipe:0: Invalid data found when processing input