I had a problem with voice mail volume in .WAV files and found a fix, but it is probably unique to our configuration. We're using Dialogic DMG2000 gateways (as PSTN gateways between our PBX and 3CX) and all voice mail .WAV files left by calls via the gateway had very low volume. Voice mail left by one 3CX extension call to another 3CX extension call had acceptable volume, so the problem was only in calls via the gateway. The Dialogic has a configuration parameter (DSP Settings/TDM to IP AGC Enable) which supports enabling automatic gain control on voice to SIP. Once I enabled this parameter the voice mail .WAV files had an acceptable volume level. If this is happening only with voice mail left via a call from a gateway, you might look for an auto gain control setting in your gateway.