The simplest method is to actually take the recordings that are currently on Elastix (asterisk), download them to your PC, and use a product called Wavepad. This has an option for reducing the volume. Save the file back out and re-upload to your ELastix box.
We have done this in the past and works well.
Alternatively, in the 1.4 Elastix coming soon, Freepbx 2.5 has the ability under the MOH Gui to reduce the volume of the Music On Hold, via a drop down box in increments of 25%.