Form is only as render-intensive as you make it, there are tons of ways to make it render in near realtime if you limit the numbers of particles or threads and are creative with the effect.
Not only that, but any effect in AE can be made sound-reactive.
1. import audio, select audio track
2. Animation -> keyframe assistant -> convert audio to keyframes
3. You now have an hour-long track of keyframes, take any effect, enable expressions, and pickwhip the effect parameters to the audio keyframe track
4. One place to start would be fractal noise parameters to animate, or size / position keyframes on shape layers, or shape layer repeater effects
5. look up the linear(x, x, x, x) expression - this will conform, say -360 to 360 degree rotational values into a 1-100 number scale that the audio keyframes work with