Buttons for slow- and fast-playback

I need to allow users to manipulate playback speed for audio; to do so, they can use three buttons:

  • - to slow down the playback speed by 10%;
  • + to speed up the playback speed by 10%;
  • = to set the playback speed to its normal 100%.

Tooltips are also used to convey users with meaning of each button. The users can also use keyboard shortcuts for the same effect:

  • Ctrl + down;
  • Ctrl + up;
  • and Ctrl + =.

But now, as I'm preparing the app for prime time, I'm looking for a better signage. Here's what I came up with (and why I think it won't work):

  • -10%, +10% and 100%: this might be confused with volume controls (of which there are none in this up);
  • ←, → and —: this is not right; slowing down a track does not make go backwards;
  • ▾, ▴ and —: the ▴ is too similar to the load media button (which the app doesn't have).

Is there any standard imagery associated with such a functionality and that we can use in this particular context?

UPDATE

Here's what I made so far (note the diamond button; I find it more appropriate than the dash, as it shows an up- and a down-button stacked on top of each other and canceling each other's effect):
enter image description here

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