The Youtube TOS doesn't allow to do so. See https://www.youtube.com/static?template=terms. It's only for streaming, not for copying or recording. So I'm locking this thread.
I'm fed up with downloading online software which infected malware or spyware programs. I'm in need of converting YouTube to MP3 which is more popular with portable devices. I need some helps for seeking simple and safe MP3 converters.