When is it appropriate to autoplay music or video on a web site?

If its an LP and the video is above the fold with an abvious pause/stop button then you might be able to get away with it. I've been admonished for coding an auto play video. My advice, it to let the user decide when to hit the play button. Your conversions will likely improve.

I tend to open links in tabs, browsing through status updates and tweets for all the articles I want to investigate further. I HATE having to find the tab that finally loaded, buffered, and started playing, even if I could have predicted it was a movie trailer, a band's website, or any of the other wrong answers given here. I am not saving a click, I'm forced to take several extra steps to find your bad work and immediately shut it down never to return. Pushing sound into a space I expect to be silent is no better than pushing pop-up ads. Users will avoid you and your site at all costs. The answer is an unqualified* NEVER. Do not ever do this, never ever. Never. *Silent video is okay. Many sites effectively use a silent, looped screencast to demonstrate software. It automatically starts looping when someone visits, but the key is that there is no pushing of content; the content is only there when you are intentionally consuming it.

I don't think the answer is never. I think it depends entirely on what the user's expectations are. If you website is titled "Guile's Street Fighter Music on Loop", then I'd say users should know what to expect. If it's a website for an upcoming film and the trailer's just been released, then yeah, go nuts with a clip that auto starts. If it's a website for your favorite band (or even an obscure one you're researching), then that's the product they're selling. Save yourself a click. If the whole purpose of the site/page is to be an immersive medium (such as Ellie Goulding's Lights: http://lights.elliegoulding.com/), then I think it's acceptable as long as the volume isn't blaring. But when the user isn't expecting it, that's where you have a problem. For some reason, someone along the way decided that users expected all restaurant websites to be in Flash and have elevator music playing. And we've all suffered as a result.

How often have you clicked on a web site and thought, "If only there were music suddenly blaring at me right now, this would be awesome?"  I'm guessing 0.  While I think has the most scientifically sound answer, I say leave the auto-play soundtracks (and flash movies) in the past.