Yes, you can use an audio track as the side chain source audio signal. A MIDI track can be the source too, but it must be sent to a bus.
1. On Audio Track 1 place a drum or rhythmic apple loop.
2. On A Soft Inst track open an ES2.
3. Select a preset you like.
4. At the top of the ES2's plug-in window click on the box next to the word side chain and select [i]Audio 1[/i].
5. In the Router section select Cutoff 1 as the Target.
6. Via is Off.
7. Source is Side Chain.
8. Raise the fader on the router to the maximum.
9. Make sure the little b/p button at the top of the router is NOT highlighted.
10. Create a cycle area around your apple in the Arrange and make sure the ES2 track header is selected.
11. Press Play and play some notes. You should now hear the sound of the ES2 being affected by the drum loop.
Does that work for you?