At normal playback volume, the 15 result is barely audible. As is, the low-gain source files strike me as being at a reasonable BG level.
Perhaps your sound system is cranked up to compensate, leading you astray.
VP volume can be lowered by moving the slider at the left of the audio track. 
Tested an export using version 8.45. The 15% setting still had a pop at about :55 - and likely others. Export was much quicker with v10x.