I've got a couple of IP670 users that sound pretty quiet when speaking on the phone. They are naturally quiet speakers in the first place.
Site has a mix of IP670 and IP335, all running on 4.0.4, talking to an Asterisk 11.4 (FreePBX distro) PBX.
These are the only two users with this issue. I'm reading about the voice.gain.tx.handset parameter, and considering tweaking its value for these 2 users. No deviance from default settings on that parameter to date. I've never had to do this before.