I'm afraid this is something I'm not entirely familiar with, but if I'm reading the Sony manual correctly, Page 35: About the BRAVIA Sync features (for HDMI connections only) I do not see any reference to controlling volume. Furthermore, the troubleshooting section, Page 31: BRAVIA Sync (“Control for HDMI”) states that if you've connected the TV via the receiver: if the AV amplifier (receiver) is not compatible with the “Control for HDMI” function, you may be unable to control the TV from the player.

Note that in all instances, it is the TV that's being controlled not the receiver, so whatever the above I think it's unlikely you be able to control the receiver volume from the Sony remote.