Highlight the audio layer. Don't use mp3 audio. Do an audio preview with the decimal point key on the numeric keypad. Don't use mp3 audio. Hit the asterisk key on the numeric keypad where you want a marker. Don't use mp3 audio. Change the duration of audio previewing in AE preferences if necessary. Don't use mp3 audio.
You'll want to listen very closely several times to figure out where in the music you want those markers to go BEFORE you do it for real. It can be a real pain to slide a boatload of layer markers off the layer if you make massive mistakes.
There's also any number of audio related scripts on AEScripts.
This is exactly what my After Effects script Beat Assistant is designed for
It can detect the bpm of your audio, generates markers on beats and can also generate keyframes and stagger layers in sync with the beat.
How does Beat Assistant handle music with rubato or tempo changes, Mathias?
What determines the user's definition of "the beat", e.g. bass guitar vs. syncopated drums vs. ostinato solo flute, where the real beat may lie?
The beat detector of Beat Assistant tries to predict one constant BPM. If the beats per minute change over time, the result will be wrong (at least for some part of the music). However, if you know for example, that a first part has 80 BPM and second part 90 BPM, you can enter the BPM manually and first set the workarea to the first part to generate markers with 80BPM and then set the workarea to the second part and generate markers for 90 BPM.
Can I use an mp3 audio?
Can I use an mp3 audio?
NO. DO NOT use mp3 audio. Convert it to a wav or aiff file first.
There are all kinds of variants of mp3 compression. Many of them don't work with AE. It's best to avoid the issue entirely right from the get-go and use uncompressed audio.
Of course, you can take your chances with an mp3, but if it's the wrong kind you'll hate yourself later.
If you have Adobe Audition, part of the CC package, then that's the tool you should be using to add markers to an audio track. Much better tool, faster to use, and more accurate. There are lots of tutorials available directly from the Audition help files that show you exactly how to add and use markers.
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BeatEdit for Premiere Pro (see BeatEdit for Premiere Pro - aescripts + aeplugins - aescripts.com ) has a much better beat detector than Beat Assistant for After Effects (see http://aescripts.com/beat-assistant/ ). When you generate clip markers in BeatEdit for Premiere Pro and after that import the audio file in After Effects, it will show the markers in the After Effects timeline, too.
And yes, it works well with both mp3 and wav files. If you only need markers and have Premiere Pro installed, too, BeatEdit for Premiere Pro is the much more robust and accurate solution.
See here how to create beat markers in Premiere with BeatEdit