I was hoping to see this pushed out by default because I agree that it's the best 'new' feature I've found in the browser in some time. You can change it in about:config, searching for 'media.autoplay'. I've changed the following:

1) Set 'ask permission' to true

2) Set 'block webaudio' to true

3) Set default to 2, which is 'ask per domain'. As I understand, the default of 0 is 'autoplay', 1 is 'block' and 2 is 'ask' but your favourite search engine of choice can provide clarification on this and the other settings. 2 will cause a pop-up beside the URL bar, like with password save popups, letting you accept or block and with a checkbox to set it permanent.