Almost all video recorders used to have an audio dub, but you must use a hi-fi machine for the best results. These machines have a high effective tape speed and the noise level is low. I have even 'bounced' the recording between machines to overdub several times with no apreciable loss of quality. It's not like cassette tapes with their low tape speed and hiss. Be sure to use manual level control though, and a decent microphone if possible. Remember, all professional recording was done on analogue tape in the pre-digital age, and because the machines used high tape speeds most of the old recordings still sound pretty good today.