Often, one has Audio that is too loud, or too soft. Sometimes, they have added a music Track, that needs to drop down to clearly hear the dialog from another Clip. While there are Audio-editing programs, like Audition, SoundBooth, and even the free Audacity, PrE, or PrPro can handle such adjustment just fine. These adjustments can be made for the entire Clip, or can be Keyframed over time.
First method, and not one that I recommend, is to adjust the Audio Gain of the Audio Clip. This is done by either Rt-clicking on a separate Audio Clip, or by first Alt-clicking, and then Rt-clicking on a Muxed Audio Clip (Multiplexed with both Video and Audio in one file). Then, one would choose Audio Gain, and adjust that. This method will also allow one to then adjust the Volume by a greater amount, and has the bonus of increasing the Waveform Dislay vertically.
The next method is to look at the Audio Clip (or the Audio portion of a Muxed Clip), and click-drag on the yellow-orange “rubberband” for Volume, and move it up, or down. This can be Keyframed, but the next method is much better and more precise, in my estimation.
My first choice is to adjust the fixed Effect>Volume. This is done from the Effects Control Panel, and it is very easy to Keyframe these adjustments over time. This is much more accurate, and actually easier, than click-dragging the rubberbands in the Clip itself. Accessing the Effects Control Panel is slightly different, from PrE to PrPro. In PrE, one would Select the Audio Clip, then go to the Effects Tab, and choose Edit Effects, locating the Volume Effects and twirling it open to view the slider. In PrPro, if one does not already have the Effects Control Panel in a Pane in the GUI, it would be accessed from Window>Effects Control Panel. With the Audio Clip Selected, one would locate the fixed Volume Effect, twirl that open, and make the same adjustments, including Keyframes, if necessary. These will be Clip Keyframes, and will follow that Clip, where ever you move it. This is a difference from the next method.
My other choice would be to use the Audio Mixer (Window>Audio Mixer, if not open), and to automate the Volume settings. This is accomplished by playing the Timeline and adjusting the sliders for Volume for each Track. In PrPro, one would first set the Send to Write, or Touch, to make those adjustments, then switch back to Read, when done. There are no adjustable Sends in PrE, so one is always in Read, until adjustments are made, and then it’s set to Write, but you will not see this change. After you are done, you can playback the Timeline, and watch the slider(s) automatically move. These will be Track Keyframes, and will remain on the Track, regardless of what Clips you add/remove from that area of the Track. Unlike the Clip Keyframes, these will NOT follow the Clip.
One can toggle the Keyframe of an Audio Track between Clip, or Track. Also, the default frequency of Audio Track Keyframes in PrE & PrPro is a bit high, in my opinion. I drop these down a bit in Edit>Preferences.
For the Keyframing, one can alter the Velocity of the Keyframes to Bezier from Linear, and the adjustments will be a bit smoother. Besides the Bezier, one can use Ease-Out and Ease-In, which are Bezier, but basically in a Preset. In PrPro, one has several different Bezier options. In PrE, one is a bit more limited.
For setting the Volume, I recommend that one only glance at the Levels Meters (unless they are peaking in the red), and instead use a pair of high-quality headphones (noise canceling models are worth the money), or fully calibrated studio monitors. Let your ears make the decisions for you, as the dB settings should only be a starting point.
In PrPro, one can also Rt-click on the Audio Clip, and choose Edit in Audition (if you have it installed), or Edit n Soundbooth (if you have one of the suites).
Here is a look at Audio Keyframes in PrPro 2.0. Things are similar in PrPro later versions, and also in PrE, with the above noted differences: