Anyone ?

Hi Chrislongski, my guess would be they are not using Flash.

Firefox has built-in HTML5 audio and video players. To see what audio formats Firefox can play, you can experiment with the following test page:

http://hpr.dogphilosophy.net/test/

Now... how can you stop autoplay of HTML5 audio? Maybe this setting:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste autop and pause while the list is filtered

(3) Double-click the media.autoplay.enabled preference to switch the value from true to false

With that setting, you're likely to encounter something odd on video sites that autostart their videos (like YouTube): the player has a pause button, has started buffering, but isn't playing. To start the player, click pause, then click play.

Unfortunately, I don't know of a way to block only some kinds of media autoplay without an add-on (which you could try searching for in case there's something new up there).

By the way, since sites playing unwanted audio are not that common, I find it more convenient to mute their tabs by clicking the little speaker icon that appears on the tab up on the tab bar. That way, I can leave autoplay on so video players behave normally.

See: Mute sound in Firefox tabs