Is there such a thing? I'd like to be able to watch old non-Hifi videotapes without or at least with significantly reduced hiss on the soundtrack.

Just for sound? That’s what graphic equalizers are for. If it’s VHS tape specifically just turn down the treble.

I would start with the source. Get the best vcr you can. There isn’t much else you can do without changing the sound significantly. If it really annoys you, you can cut the treble a little but it will result in loss of sound stage and air.

What you want is likely available for post processing on a computer but not on the fly in real time.

I take that as a "no" then. I figure that even if you could do it in real time, it'd introduce some lag into the audio stream unless the post-processor routed the video and lagged that down as well.

In which case I'll move on to the next question; is there any way I can extract the audio from a DVD and then re-insert it without having to re-encode the whole thing? I converted some old home videos to DVD but the tape hiss is really bad. I could probably cut it down quite a bit on Goldwave but first I'd have to get it out and then get it back in, too.