The sound isn’t playing, but the video is.
The ad below the News and Things sidebar is always an autoplaying video file. Using Inspect Element in Firefox shows the prime offender to be rpsadblock-2. rpsadblock-3 is sometimes an autoplaying video, but most of the time it isn’t. The contents of rpsadblock-2 for me appears to always be an autoplaying video.
I don’t know if it is due to the autoplaying videos, but loading the RPS mainpages also trigger a resource hog Flash process that starts at around 300MB of memory allocation and can eventually rise above 700MB. It also will randomly bring Firefox to a crawl, before popping up the message box about a script on the RPS page not responding.
(EDIT: I don’t know that the slowed Firefox and unresponsive script is entirely the fault of the these specific video ads. To me, the RPS main site has long shown poor performance compared to other web sites, which I’ve often blamed on the types and number of ads that RPS runs. Sometimes it was extremely obvious that certain ads were a real problem. Heck, RPS is why I started using flash and site blockers and the like in the first place. But I cannot say that the video ads alone are the issue.)