If there is not an existing Audio Mix effect on the clip before you start, you do not hear any changes until you click the Audio Loop Play button to stop and replay the effect.

As you adjust the volume values on an existing Audio Mix effect, you might not hear the results immediately. It takes a few seconds for the Avid system to apply the changes to the clip. The response time for this feature is considerably longer than it is when changing EQ parameters while using Audio Loop Play. See Adjusting Audio Latency. You might need to click the Audio Loop Play button to complete the edit and then play the effect to hear the result.