Scrubbing and Seeking
From Audacity Manual
Once you have placed Audacity in scrub play mode it will remain in that mode until you Stop (or use a shortcut to stop) which would then allow you to restart in normal play mode.
The term "Scrubbing" comes from the early days of the recording industry and refers to the process of physically rotating tape reels to move the tape past the playhead to locate a specific point in the audio track.
Scrubbing can only be started when in Selection Tool mode. Changing to another tool after starting scrubbing will perform a Stop.
Pressing CTRL + left-click will put you into scrub play mode. You can now release the mouse button then move the pointer right and left across the waveform. This will cause Audacity to scrub play forward or backward respectively from the point at which you clicked in the waveform. Nothing will happen until you move the mouse pointer. Scrub play will temporarily stop when the green playback cursor (the "playhead") catches up with the mouse pointer but will restart again as soon as you move the mouse left or right.
The speed of scrub play can be controlled by rotating the mouse wheel (if your mouse is so equipped). Each four steps upwards of the rotated mouse wheel doubles the speed, and each four steps downwards halves the speed. This is equivalent to one octave of pitch change. The changed scrub speed will be shown temporarily in yellow-ochre superimposed on the track.
It is possible to zoom while you are scrubbing by holding the CTRL key down and rotating the mouse wheel.
Clicking the Stop button (or the shortcuts SPACE to stop at the editing cursor or SHIFT + A to stop where playback ends) will stop scrub play and let you restart normal playback from the stop point. To mark a position found during scrub you can also use the Stop button or SPACE to stop when the green playback cursor is over the mouse pointer. Then left-click to move the editing cursor there and use CTRL + B to create a label at that point.
You can invoke scrub play while audio is playing normally and it will abandon the playback to perform scrub play. You cannot invoke scrub play while you are recording or paused in recording mode.
If you hold the left mouse button down as you scrub the scrubbing will change to seeking.
In seek mode many small slices of the audio are played (at normal speed even if you have changed speed). The playhead is always moved very close to the mouse pointer enabling you to move across the audio rapidly. This is similar to using the seek button on a CD player.
Releasing the left mouse button while seeking will return you to scrub play and to any changed speed you had selected. Holding left mouse button again will return you to seek play for quick navigation. With practice you will find it easy to repeatedly hold then release the left mouse button to switch between seeking and then scrubbing to focus on a particular area.
A single left-click anywhere in the waveform while scrubbing mode will immediately seek to that position. Scrubbing will not resume until you move the pointer again.
If you CTRL + double-left-click then move the mouse pointer to scrub, the green play cursor (effectively the "playhead") remains at the mid-position of the waveform display and the speed of scrubbing can be varied much more smoothly by moving the mouse pointer.
In this mode the mouse pointer position does not determine the target location of the scrub, but determines the speed of the scrub. The further away you move the mouse pointer from the play cursor (in either direction), the faster the speed of the scrub. The current scrub speed is shown permanently in lime green, superimposed on the track as you move the pointer.
The speed of the scroll-scrub as determined by the mouse position can be varied by rotating the mouse wheel (if your mouse is so equipped). Move the wheel upwards to increase the speed or downwards to reduce the speed. This works at the same time as modifying the speed by moving the pointer left or right.
When scroll-scrubbing backwards, as play approaches the leftmost portion of the audio the green play cursor will move from a centered position to the left until it reaches zero and stops. If you turn on "Enable Scrolling left of zero" in Tracks Preferences then the play cursor will remain centered as you approach the leftmost portion of the audio. Enabling this preference makes it easier to scroll-scrub at slow speeds close to the start of the track.
If you hold the left mouse button down as you scroll-scrub, scroll-scrubbing will change to scroll-seeking. The modified speed when scroll-seeking is indicated by an "X" at the end of the speed displayed in lime-green.
Releasing the left mouse while you are scrub-seeking will return you to scroll-scrub play and holding it again will return you to scroll-seek play.
If you quickly press and release the left mouse button when scroll-scrubbing, this jumps the time position of the midline in the current direction of scroll-scrub. The jumps are longer, the further the pointer is from the midline.