Click and drag the connector line downwards. As you drag the line down, a small context menu appears, the number values in this menu are decibel levels. Negative numbers indicate a lowering of volume. Pull the connector line down to a level of -12.73 dB. Your numbers may be slightly different.

Note
In Premiere Elements. volume is measured in decibels. A level of 0.0 dB can be thought of as the original volume (not silence). Changing the levels to a negative number reduces the volume, changing the volume to a positive number increases the volume.