FFmpeg error while converting from MP4 to MP3 in Python

When I try to convert videos from a YouTube playlist, it gives me an error:

File "C:\Users\fonti\AppData\Local\Programs\Python\Python310\lib\site-packages\ffmpy.py", line 106, in run raise FFRuntimeError(self.cmd, self.process.returncode, out[0], out[1]) ffmpy.FFRuntimeError: C:/ffmpeg/bin/ffmpeg.exe -i "Nightcore - To Be Human // lyrics.mp4" "Nightcore - To Be Human // lyrics.mp3" exited with status 1

The code I'm using:

from pytube import Playlist
import ffmpy
from ffmpy import FFmpeg

playlistLink = input("Introduz o link da Playlist: ")
playlist = Playlist(playlistLink)   

diretório = 'E'
while diretório != 'M' and diretório != 'V':
    diretório = input("Vais baixar música ou vídeos? (M/V)")
    if diretório == 'M':
        downloadDirectory = "C:/Users/fonti/Documents/Projetos Python/Youtube/Músicas"
    elif diretório == 'V':
        downloadDirectory = "C:/Users/fonti/Documents/Projetos Python/Youtube/Vídeos"

print("Número total de vídeos a baixar: ", len(playlist.video_urls))    

print("\n\n Links dos vídeos:\n")

for url in playlist.video_urls:
    print(url) 

def MP3():
    for video in playlist.videos:

        audio = video.streams.get_audio_only()
    
        audio.download(downloadDirectory)

        videoTitle = video.title
    
        new_filename = videoTitle + '.mp3'
        default_filename = videoTitle + '.mp4'
    
        print(default_filename+'\n\n'+new_filename)

        ff = ffmpy.FFmpeg(
            executable = 'C:/ffmpeg/bin/ffmpeg.exe',
            inputs={default_filename : None},
            outputs={new_filename : None}
        )
        ff.run()


def MP4():
    for video in playlist.videos:
        print('Downloading : {} with url : {}'.format(video.title, video.watch_url))
        video.streams.\
            filter(type='video', progressive=True, file_extension='mp4').\
            order_by('resolution').\
            desc().\
            first().\
            download(downloadDirectory)

escolha = 'E'
while escolha != 'V' and escolha != 'A':
    escolha = input("Queres formato de vídeo ou áudio (V/A)? ")
    if escolha == 'V':
        MP4()
    elif escolha == 'A':
        MP3()
    else:
        print("Escolha inválida")

If I try to download the videos from the playlist, it works fine. But when I try to download the audios, it gives me the error.

We may use full path so FFmpeg can find the files.
We may join downloadDirectory with videoTitle for creating full path:

def MP3():
        ...
        videoTitle = video.title

        videoTitle = os.path.join(downloadDirectory, videoTitle)

        new_filename = videoTitle + '.mp3'
        ...

Complete code sample:

from pytube import Playlist
import ffmpy
from ffmpy import FFmpeg
import os

playlistLink = 'https://www.youtube.com/playlist?list=PLS1QulWo1RIaJECMeUT4LFwJ-ghgoSH6n' #input("Introduz o link da Playlist: ")
playlist = Playlist(playlistLink)   

diretório = 'E'
while diretório != 'M' and diretório != 'V':
    diretório = 'M' #input("Vais baixar música ou vídeos? (M/V)")
    if diretório == 'M':
        downloadDirectory = "C:/Users/fonti/Documents/Projetos Python/Youtube/Músicas"
    elif diretório == 'V':
        downloadDirectory = "C:/Users/fonti/Documents/Projetos Python/Youtube/Vídeos"

print("Número total de vídeos a baixar: ", len(playlist.video_urls))    

print("\n\n Links dos vídeos:\n")

for url in playlist.video_urls:
    print(url) 

def MP3():
    for video in playlist.videos:

        audio = video.streams.get_audio_only()
    
        audio.download(downloadDirectory)

        videoTitle = video.title

        videoTitle = os.path.join(downloadDirectory, videoTitle)  # Concatenate the directory and the file name
    
        new_filename = videoTitle + '.mp3'
        default_filename = videoTitle + '.mp4'        
    
        print(default_filename+'\n\n'+new_filename)

        ff = ffmpy.FFmpeg(
            executable = 'C:/ffmpeg/bin/ffmpeg.exe',
            inputs={default_filename : None},
            outputs={new_filename : None}
        )
        ff.run()


def MP4():
    for video in playlist.videos:
        print('Downloading : {} with url : {}'.format(video.title, video.watch_url))
        video.streams.\
            filter(type='video', progressive=True, file_extension='mp4').\
            order_by('resolution').\
            desc().\
            first().\
            download(downloadDirectory)

escolha = 'E'
while escolha != 'V' and escolha != 'A':
    escolha = 'A' #input("Queres formato de vídeo ou áudio (V/A)? ")
    if escolha == 'V':
        MP4()
    elif escolha == 'A':
        MP3()
    else:
        print("Escolha inválida")

More robust solution, is searching for the .mp4 file in the folder, and delete it after converting to mp3:

from pytube import Playlist
import ffmpy
from ffmpy import FFmpeg
import os
import glob

playlistLink = 'https://www.youtube.com/playlist?list=PLS1QulWo1RIaJECMeUT4LFwJ-ghgoSH6n' #input("Introduz o link da Playlist: ")
playlist = Playlist(playlistLink)   

diretório = 'E'
while diretório != 'M' and diretório != 'V':
    diretório = 'M' #input("Vais baixar música ou vídeos? (M/V)")
    if diretório == 'M':
        downloadDirectory = "C:/Users/fonti/Documents/Projetos Python/Youtube/Músicas"
    elif diretório == 'V':
        downloadDirectory = "C:/Users/fonti/Documents/Projetos Python/Youtube/Vídeos"

print("Número total de vídeos a baixar: ", len(playlist.video_urls))    

print("\n\n Links dos vídeos:\n")

for url in playlist.video_urls:
    print(url) 

def MP3():
    # Delete all mp4 files in Músicas folder
    mp4_files = glob.glob(os.path.join(downloadDirectory, '*.mp4'))  # dir C:/Users/fonti/Documents/Projetos Python/Youtube/Músicas/*.mp4
    for f in mp4_files:
        os.remove(f)

    for video in playlist.videos:

        audio = video.streams.get_audio_only()
    
        audio.download(downloadDirectory)

        videoTitle = video.title
        videoTitle = videoTitle.strip() # use the strip() method to remove trailing and leading spaces https://stackoverflow.com/a/10443548/4926757
       
        # List all mp4 files in the directory.
        videoTitle = glob.glob(os.path.join(downloadDirectory, '*.mp4'))  # dir C:/Users/fonti/Documents/Projetos Python/Youtube/Músicas/*.mp4
        
        #videoTitle = os.path.join(downloadDirectory, videoTitle)  # Concatenate the directory and the file name
    
        #new_filename = videoTitle + '.mp3'
        #default_filename = videoTitle + '.mp4'

        default_filename = videoTitle[0]  # Suppused to be only one mp4 file in the folder.
        new_filename = default_filename.replace('.mp4', '.mp3')  # Remplace .mp4 with .mp3
    
        print(default_filename+'\n\n'+new_filename)

        ff = ffmpy.FFmpeg(
            executable = 'C:/ffmpeg/bin/ffmpeg.exe',
            inputs={default_filename : None},
            outputs={new_filename : None}
        )
        ff.run()

        os.remove(default_filename)  # Delete the mp4 file


def MP4():
    for video in playlist.videos:
        print('Downloading : {} with url : {}'.format(video.title, video.watch_url))
        video.streams.\
            filter(type='video', progressive=True, file_extension='mp4').\
            order_by('resolution').\
            desc().\
            first().\
            download(downloadDirectory)

escolha = 'E'
while escolha != 'V' and escolha != 'A':
    escolha = 'A' #input("Queres formato de vídeo ou áudio (V/A)? ")
    if escolha == 'V':
        MP4()
    elif escolha == 'A':
        MP3()
    else:
        print("Escolha inválida")