Transcription Toolbar
From Audacity Manual

Transcription Toolbar has its own own Play 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, audio only plays at normal speed if using Space or the Play button on the Control Toolbar to play.
Drag the slider to set a speed from one-hundredth of normal (0.01x) to three times as fast (3.00x), or 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 - you can adjust the slider position during play but this will restart playback from the cursor position.