Thank you for the reply.
I've spent about 20 hours testing various extensions in various combinations trying to find perfect audio/video blocking and click-to-play.
The only perfect blocking I've found is NoScript using its "Allow Scripts Globally", "Forbid Audio/Video", and "Apply these extensions to whitelisted sites too" checkboxes. Its click-to-play functionality isn't as good & aesthetic as Flash Control, but it can handle things like Soundcloud. The way it handles it is that the NoScript addon-bar icon changes to indicate that blocking is happening, then you can right-click it, go to Blocked Objects and then Temporarily Allow. What I don't like about NoScript is the number of clicks it takes to unblock in this scenario, and that it has to reload the page to unblock, and reloading the page again doesn't re-block, and temporarily unblocking one page unblocks the entire site for the duration of the browser session or until you do a "Revoke Temporary Permissions".
I tried hybrid solutions like using NoScript for audio (since it works consistently despite being inelegant) combined another more elegant extension like Flash Control or Flashblock for video, but NoScript doesn't let you independently block audio or video, it's both or neither.
So in terms of audio/video blocking and click-to-play, Flash Control is better in some ways and NoScript is better in other ways. I'm going to try using only Flash Control for a while to see how many other sites like Soundcloud I run into.
I made a contribution because I think you're on the right track.