You can use the convert audio to keyframes command under animation>keyframe assistant. This will give you keyframes relating to to the volume of the audio which you can link to via expressions. For more advanced abilities, like being able to isolate certain frequencies, try soundkeys by trapcode. There are tons of tutorials around to do this, just do a quick google search with some of these terms.
Thanks, that really did help a lot! Appreciate it
Also see these tutorials to learn how to use this approach with iExpressions:
Also, Beat Assistant can be useful to animate in sync with the music. The results will feel more regular, accurate and less fuzzy/organic than with expressions/iExpressions.
Finally, Red Giants Sound Keys is pretty useful for these kinds of jobs:
Yes, all very creative and interesting variations. That's very helpful of you