The challenge with Smart Mix is that, unless you undo it right after you create it, the only way to remove it is to manually delete the keyframes it created.
What method have you used so far?
Assuming you have Premiere Elements 11: Go to Expert View, select the audio (portion of) clip that has the key frames on it. select the "Adjust" button in the top-right side of the app, next to the monitor. Here open the keyframes view by clicking on the little "Show/Hide Keyframes Window" button (marked "1" in the screenshot) on the top right of the window (to the right of the name of the Window which should be called "Adjustments). Once this view is expanded, click on the "Toggle Animation" button (marked "2" in the screenshot) which should look pressed again near the top of the window. This will prompt a message box that asks if you want to delete all key frames. Say OK here and you should be done.
The method is similar in previous versions of Premiere Elements as well, but is placed in "Applied Effects" panel I think.
Here is a screenshot of the button positions.