There are no gain adjustments on the Cisco Unity Express module itself. The only options are to adjust signal level on the voice-ports itself.
In order to increase the signal strength that comes in from the public switched telephone network (PSTN), you can use the input gain on the voice-ports. If the voice level is too low, you can use the input gain command in order to increase the input gain.
configure terminal voice-port XXX <--- appropriate voice-port input gain 3 shut no shut
If you access voicemail remotely, then you can use the output attenuation command under the voice-port on the router in order to increase the volume of the signal that leavies the router to the PSTN.
configure terminal voice-port XXX <--- appropriate voice-port output attenuation -3 <---this would increase the outbound volume by 3db. shut no shut
-3 db is a good place to start. If it is too loud, try -2, and if it is nott loud enough, try -4. Try to avoid to go more than -6 as you can amplify line noise and cause other issues.