Always good to give the computer operating system involved in case any computer specific details needed to be included. But here is your
generalized yet specific answer for Premiere Elements 12 and increasing the volume of a video clip.
If you are in the Expert workspace...
a. click and drag upward the orange band that runs horizontally through the middle of the audio clip.
b. go to Adjust Tab/Adjustments Palette/ Volume Panel expanded on move the volume slide toward +6 dB.
0 db does not represent no sound. It represents the volume of the sound when it was recorded. 0 to +6 dB increases the sound above the recording level; 0 to infinity symbol decreases the sound below the recording level, finally to muted sound.
Please let us know if you are OK with the details above. My blog post on PE12: Audio Features may be useful to you.