There are two main ways to control the sound levels in Adobe Premiere Pro over time outside of a simple constant gain, expositional fade transition.

Firstly, it is not straight away that obvious that this can be done in the timeline in much the same way as in Final Cut Pro and the use of pushpins.

Locate the button to the left of the timeline and highlight it. You should now be able to place pushpins along the levels line.

Place the playhead where you would like your fade or gain to begin and then click the grey diamond where you see the text “Audio” to the left of the timeline. You will notice a pushpin / keyframe appear on the audio track and the grey diamond turn black.

Move the playhead and the diamond will turn grey again, place the playhead where you would like your fade to finish and add another pushpin / keyframe in the same way. Grab the level beyond your second pushpin / keyframe and drag it down.

You have created a gradual fade over time.

The second way is much the same as creating motion with images and takes place in the effects tab. If you have the Show Clip Keyframes drop down selected, you will notice this in the timeline to. With your clip selected place your playhead where you would like the fade to begin.

Expand the levels controls under effects and locate the stopwatch symbol. Press this and a small black diamond (keyframe) will appear in the clip timeline. Drag the playhead to the end of the fade and repeat. You should now be able to reduce the levels beyond your second keyframe without affecting the sound before the first. The diagonal fall between the two signifies the fade over time.