Hi Peter,
You can use the Batch Converter to do this. Just select the folder containing the files you want to change (I would recommend working on a copy of the folder to start with, or making a backup), and then add an 'Amplify' to the command list. Choose a level less than 100% to reduce the volume.
Of course, there may just be a setting in your flight sim to turn down the chatter volume, but I guess you've already exhausted possibilities like this. I'm pretty sure X-Plane has a control panel to adjust the relative level of different sounds.
Alex