Audacity is a free, open source audio program that is compatible with a number of operating systems. Use it to clean up your audio for PLR videos. It is a versatile product to help your PLR videos stand head and shoulders above others on the market in your
niche. Wavepad has a free and a paid version and also works well.
Audacity - http://www.audacity.sourceforge.net
WavePad - http://www.nch.com.au/wavepad/