control pvr volume instead of tv volume

[Zegma]

Hi

i hope someone can help

i'm trying to set up a system to control the HD PVR's volume instead of the tv volume alone

obviously when they are on dvd or anything else it must control the tv volume

but when they change to watch thier DSTV they want to control the PVR volume this is because the audio is being fed to a sonos system which is kept at full volume because there is no sonos control through control4

extra info about the system the pvr signal is fed to the room through RF cable and then converted to a video and audio signal

Thanks in advance