How to convert youtube video into audio file?

While many third-party websites and software offer to convert YouTube videos to audio, most of these methods violate YouTube's Terms of Service. The legality is complicated and can depend on the copyright status of the content and the user's intent. The safest and most reliable methods to get audio from YouTube involve using its own official services or creating your own content. 
Using official, sanctioned methods
These options let you listen to or download audio legally through YouTube's own ecosystem.

• For offline playback (YouTube Premium)

A YouTube Premium subscription allows you to download videos for offline viewing within the YouTube mobile app. While it doesn't create a separate audio file, you can listen to the audio with your device's screen off, effectively turning it into a music or podcast player. 

• For your own creations (YouTube Audio Library)

If you are a content creator looking for music or sound effects, YouTube provides a library of free, high-quality audio that you can download and use legally. 

• Navigate to the YouTube Studio and find the Audio Library in the side menu.
• Use the search and filter options to find tracks by genre, artist, mood, and duration.
• Hover over the date column and click the Download button to save the audio file.