After this post, some users have asked me to recommend a few solutions online, among the many solutions I found only two worthy of note.
ListenToYouTube
Simple, fast, without publicity (I use adblock;)), excellent audio quality and if your video has already been converted by other users do not even have to wait to download your MP3.
VidToMP3
Virtually identical to the previous, same service, if a server is flooded you can fall back on this or vice versa.
Other sites offer services more or less decent but too much publicity and uncontrolled downloading of unwanted applications automatically lead to being excluded from the list, as always do not forget to advise and comment.