You can use youtube-dl to download only the audio of a given youtube video’s URL.

Check the available formats of the video with the -F flag:

youtube-dl -F <video_url>

Some of the formats available will have “audio only” indicated in the listing.

Once you have the format number you would like to download use the -f to perform the actual download:

youtube-dl -f <format_number_id> <video_url>