One thing that I have found with the default voicemail configuration is that the sound files delivered to my email address have a very low volume level. If you are receiving voice mails via email to a cell phone, you can’t even hear it. To fix this and boost the voicemail to email attachment volume, do the following:

First, let’s boost the gain by going to Setup –> General Settings in FreePBX. Under the Voicemail section, change ‘Use gain when recording the voicemail message (optional)’ to 6.0. Click ‘Submit’ followed by the red bar to apply changes.
The next thing we can do is change the voicemail to email attachment format. There are two flavors of attachment…WAV and WAV49. It defaults to WAV49 to save file size, but let’s change it to WAV. From the Linux CLI, do the following:

cd /etc/asterisk
nano vm_general.inc

Change

format=wav49|wav

to

format=wav|wav49

CTRL+X to exit and then Y when asked to save.

Also note in the same file the ’serveremail=’ field…this is the ‘from’ address of the voicemail emails. Change it to something that matches your organization or domain.