Sometimes they use VMED 28 or 29 but usually its VFIRE 21. I have heard sheriff deputies also mention the state SAR channel too, which is 155.16
Another reason you may not hear anything is that you are too far away from the incident. Unless patched into another frequency or channel all communications on VFIRE 21 are simplex. Usually incident command will talk with dispatch before the arrival of the air unit to let them know where to get a hold of them and vice-versa, the heli's dispatch will give them a channel to reach units on-scene at. As the heli approaches they hail incident command and/or fire dispatch and request scene details and landing zone info.