[Wishlist] A way to slow down / speed-up talk-throughs

There are 2 use cases for this:
1) If we can slow them down, then we can have non-English-language dubbings in languages with longer words.
2) If we can slow and speed up, then we can give students speed options like available on videos.

Theoretically this can be done via playbackRate and a multiplier on the JSON commands array run-through.

Start by locating the talk-through playback entry point and the JSON commands array mentioned in the issue. Determine how playbackRate and command timing relate, then define and test slower and faster playback options. Done means talk-throughs can be adjusted for language dubbing and student-selected speeds.

Written by the indexing model from the issue text.