How can I add audio to video?

Overview

Sorry for this simple question, but I couldn't figure it out.

I want to load a video, process each frame, and then write it with changes. for loading video I use open-cv, but for writing I use PyAV

I took a code from this awesome guy at Stack Overflow and the video writer works flawlessly, but I want to add audio to it.

this is my code:

width, height, fps = width, height, fps  # Select video resolution and framerate.
output_memory_file = io.BytesIO()  # Create BytesIO "in memory file".

output = av.open(output_memory_file, 'w', format="mp4")  # Open "in memory file" as MP4 video output
stream = output.add_stream('h264', str(fps))  # Add H.264 video stream to the MP4 container, with framerate = fps.
stream.width = width  # Set frame width
stream.height = height  # Set frame height
stream.pix_fmt = 'yuv444p'   # Select yuv444p pixel format (better quality than default yuv420p).
stream.options = {'crf': '17'}  # Select low crf for high quality (the price is larger file size).

# Done breaking down video to pictures and making them pixeled, now making it videos back.
# imgs is a dict with numbered frames as PIL Image

for i in range(len(imgs)):
    frame = av.VideoFrame.from_image(imgs[i])
    packet = stream.encode(frame)  # Encode video frame
    output.mux(packet)  # "Mux" the encoded frame (add the encoded frame to MP4 file).

# Flush the encoder
packet = stream.encode(None)
output.mux(packet)

output.close()

# Write BytesIO from RAM to file, for testing
with open(video_name, "wb") as f:
    f.write(output_memory_file.getbuffer())

can anyone help me?