Transcription Toolbar has its own Play-at-Speed button which plays audio at the speed set by the Play-at-Speed slider to the right. Playback can be paused and resumed at the adjusted speed. Note that whatever the setting on the slider, using Space or the Play button on the Transport Toolbar to play will only do so at normal speed.
Drag the slider to set a speed from one-hundredth of normal (0.01x) to three times as fast (3.00x), or right-click on the slider then use the keyboard arrow keys to change speed by 0.03x increments. To set a precise playback speed, double-click over the slider (or create a keyboard shortcut for this action), enter the desired speed in the box and click OK.
It isn't currently possible to change speed during playback. To change speed when audio is already playing, change the speed first with the slider, then click the green Play-at-Speed button to play at the new speed. Playback resumes from the original cursor position.