For out TV, we have a Telus set-top box IPV 6015 and Slimline_2 controller. We feed our audio from the set-top box "Audio Out" connectors directly to our audio amplifier and speakers. We have always used the TV remote to Mute and adjust volume. When we got the IPV6015 box, this no longer worked with the Slimline 2 controller, so we just continued using the old controller (which I think we got about 8 years ago). It worked fine for muting and controlling volume (as well as all the normal video stuff).
That old controller has died, so we are trying to get the Slimline 2 to work the same way. When we adjust Volume or press Mute, we can now see a "slider" on the screen, changing as it should, but the sound output does not change. My son says that it _does_ work if we are use the crappy speakers built in to the TV.
Please explain how we can get the Slimline 2 to behave like the old controller in this regard.