```python
import os
from moviepy.editor import *
from moviepy.video.fx import mirror_x
import subprocess

video_filenames = ['video.mp4']
audio_filenames = ['audio.mp3']

# Check and download missing files
for filename in video_filenames + audio_filenames:
    if not os.path.exists(filename):
        try:
            print(f"Downloading {filename}...")
            subprocess.check_call(['youtube-dl', 'url_of_the_video_or_audio', '-o', filename])
        except subprocess.CalledProcessError:
            print(f"Error downloading {filename}. Exiting.")
            exit(1)

# Load and process audio
audio = AudioFileClip('audio.mp3')
audio_subclip = audio.subclip(10, 70).audio_fadein(3).audio_fadeout(3)
audio_period = audio_subclip.find_audio_period()

# Load and process video
video = VideoFileClip('video.mp4')
video_subclip = video.subclip(50, 120).crop(x1=50, y1=60, x2=460, y2=360)
looping_segment = video_subclip.find_looping_segment()

# Extract looping segment, slow down, and loop for audio duration
loop = video_subclip.subclip(*looping_segment).fx(vfx.speedx, new_duration=audio_subclip.duration)
loop_mirror = loop.fx(mirror_x)
loop_combined = clips_array([[loop, loop_mirror]]).fadein(3).fadeout(3)

# Add audio to video
final_clip_with_audio = loop_combined.set_audio(audio_subclip)

# Create title and credits screens
title = TextClip("Title of the Video", fontsize=70, color='white', size=video.size).set_duration(5).set_pos('center').fadein(2).fadeout(2)
credits = TextClip("Credits: Your Name", fontsize=50, color='white', size=video.size, bg_color='black').set_duration(5).set_pos('center').fadein(2).fadeout(2)

# Concatenate title, video, and credits
final_video = concatenate_videoclips([title, final_clip_with_audio, credits])

# Write the final video file
final_video.write_videofile("final_output.mp4", fps=24, audio_bitrate="192k", bitrate="2000k")
```