HDMI ARC to Sky but audio from different source - Denon AVR2500

I'm struggling to work out how to do this ahead of a couple of quite important cricket matches ;-)

I have a Sky box running to my Denon AVR2500 via HDMI; in turn the Denon is connected to HDMI 4 ARC on a Sony Bravia KD-55X8000C.

Having endured some awful Aussie commentators (looking at you, Steve Waugh!) I am wanting to go with alternative audio - BBC Test Match Special over 5 Live Sports Extra. This can stream to the AVR by Sonos to the optical in or line inputs, AirPlay, Bluetooth - plenty of options.

So how do I assign images to HDMI to the TV but audio from somewhere else without losing the images? Currently all I get is audio with Denon plastered all over the screen....

All suggestions gratefully received - ideally before 2130 tonight ;-)

Thanks in advance

b