One definition of "spotting" audio, as used in the film and video editing business, is the process of marking points in an audio track or region that you want to synchronize with visual events in the edit. That could be finding beats in a music track to edit to or identifying visual hit points to line up sound effects to.
Spotting audio in Adobe After Effects is easy, and kind of fun in a nerdy way, once you know how. And here's how:
Assuming we already have an audio track we want to spot (we're using a brief rhythm track here), roll down the twirly arrow (also known as the 'disclosure triangle' - Ed.) to expose the Properties for the layer with the audio, then roll down the Audio twirly, then the Waveform twirly to show the audio waveform for the layer:
(A shortcut for this is to type LL on the keyboard.)
Now, if you're familiar with reading audio waveforms, you could identify the main beats in the track and visually add markers at those points, but a better way is to add the markers in realtime while listening to the track. To do that, first make sure your audio layer is selected in the Timeline; with no layer selected, new markers will be added to the Timeline instead. Next, listen to the track by either doing a RAM preview or an audio-only preview:
RAM Preview: 0 (zero) on keypad (Mac and Windows) or Control-0 (Mac only)
Audio-only Preview (from current time): . (decimal point) on keypad (Mac and Windows) or Control-. (period) (Mac only)
(At this point, I'd recommend the Audio-only preview, because it renders much faster than the full RAM preview.)
Now, listen to the audio track to hear the main beats, and hit * (multiply or asterisk) on your keypad (Mac and Windows) or Control-8 (Mac only) as the audio plays to add layer markers at those beat points:
If you find that your markers are a little off, you can manually drag them to relocate them. And you also may find it helpful to scrub or manually drag through the audio to audibly locate hit points. To manually scrub audio, hold down Ctrl (Win) or Command (Mac) and drag the Current Time Indicator above the Timeline. With a little practice, you get used to the way the scrubbed audio sounds and can easily identify your hit points.
Now that you have your markers in place, you can use them to sync up animation keyframes or other visual events with your audio.
Check out After Effects 101 tutorial much more!