It’s your adblock software not completely blocking some ads that include autoplay content.
First off, Tumblr isn’t going to care, because Yahoo wants to make some money off this sinkhole of a site, and more and more intrusive ads are the way to do it, especially if we’re all trying to block them anyway.
However, running the site WITH adblock disabled, I could see the ads, but not mute them (pausing/muting went away if I scrolled up or down), so I’m going to say they’re being fucks anyway and don’t deserve the advertising money if they’re going to assault me with unwanted autoplay sound that I can’t stop.
Open up your ad blocker, create a custom filter for:
*.tubemogul.com*
and that should stop it. Has worked for me so far, but I literally just did this 10 minutes ago so I don’t know if it’s a complete or correct solution. If anyone’s a bigger nerd about adblock filtering or wants to dig for other sources for the autoplay ads, hit me up.