Lower the Master volume in the audio mixer.
Harm's solution will lower the actual volume of the media, including exports. From your post, I got the impression you just wanted to lower the volume while editing.
There is no way to do that within Premiere Pro. It's done externally through the speaker volume or the computer's main audio volume. Since that goes all the way down to mute, it's just not possible that you have the computer's volume turned all the way down and it's still too loud.
So, turn it down more.
Thanks Jim. Thats correct, thats what Im trying to do. Im on a mac and the step between mute and volume 1 using the keyboard shortcuts is too large when using earbuds. But your post reminded me that I can change the volume more precisely using the volume icon at the top of my screen. Thanks!