If you use Chrome I got This Disable HTML5-autoplay thing
from the Chrome Web Store and it stopped the annoying audio from plaguing my precious tumblr time.
Keep in mind I'm not a computermancer and the magic of why and how such things work is beyond me, but it DID work *shrugs*
(It is also worth noting that you have to change the option to "Disable nothing" when you WANT to listen to things, like say on youtube, but it's easy to do)
If others have better solutions I'd be super thrilled to try them. I hate these new audio ad things