as per this conversation on sipx-users:

-----Original Message-----
From: sipx-users-bounces@list.sipfoundry.org [mailto:sipx-users-bounces@list.sipfoundry.org] On Behalf Of Damian Krzeminski
Sent: Monday, February 25, 2008 2:07 PM
To: sipx-users@list.sipfoundry.org
Subject: Re: [sipx-users] volume level of calls
Matthew Kennedy wrote:
> I did find some information about changing the gain on the phone itself:
>
> http://lists.digium.com/pipermail/asterisk-users/2006-June/153809.html
>
> and I found the option I want to try adjusting in sip.cfg in
> tftproot\ "voice.gain.rx.digital.handset"
>
> before I modify its value, I'd like to test it on just one phone
> instead of every phone in the office. Which file do I need to modify
> to accomplish this? sipx-sip.cfg or sipx-phone.cfg from the phone's
> macdir? Looks like the sipx-sip.cfg but I just want to make sure before
> I do this . . . :)
>
Modify
/etc/polycom/mac-address.d/sip-2.0.cfg.vm
It is a template used to generate files in tftproot directory.
Not sure why it's not accessible from UI - you might want to open
improvement request.
D.