Hi Iain
the Tivo can output Dolby Digital surround sound via the HDMI cable, if the box is setup to do this (see instructions HERE) and the channel is broadcasting with surround sound which I think is only HD channels at present.
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Scott
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out so my answers may be wrong :-) If you do like my answer please mark it as helpful as it may help others
My setup: Vivid 200 Optical fibre with Superhub 2 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.