Lowering the gain or decibels (dB) of an audio file can be done in a free program called Audacity.

Steps

1. Load the file into the program with File > Open (or drag and drop).

2. Click on Effect > Amplify

3. Slide the Amplification (dB)* bar to the left to reduce the amplification and click OK.

4. Once you've reached the desired dB levels, click File > Export > Export as WAV to save the file. 

* The same can be done to amplify or increase the gain of the file with the Amplification (db) slider moving to the right.