And of course you're asking this after having read the help documentation on Audio, right?
I agree with Joe, in that the only way that the sliders will work on their own, is if there are Audio Track Keyframes on that Audio Track. That is done by changing the Send to something like Write or Touch. You can easily see if there are Track Keyframes there, invoking Automation, by changing the Keyframe Display to Track Keyframes. Those can be Selected and Deleted, to remove any unwanted Automation. Also, see this ARTICLE, as it goes into a bit of detail on the difference between Clip Volume Keyframes and Track Volume Keyframes. I do agree that the Help File covers the Audio Mixer well, but it does take a bit of "time with the Audio Mixer," to get all of the concepts down.
Got it! Thanks to all for your answers.