bit confusing there, but the DAV system doesnt output sound to the TV only via the speakers afaik, do you have any settings on the TV to control the outputs for sound on the back? usually its screen linked so whatever is on tv be it normal TV or via the AV channel it gets routed via the rca's on the back. on my TV I actually have to turn off the internal speakers to get the RCA's on the back to function :(
if the dav has enough inputs you can use a set of rca cables for each source.
problem with connecting the DVD to the TV via RCA leads is your not getting the best possible picture on your TV.
having just checked my TV theres an option for the scart sound output, i can set it from TV to any of the outer sources, Im asuming your getting the sound from a scart socket on the back of the TV and not a seperate set of RCA jacks on the back of the TV? if its via the scart then there should be an option to select what comes out of the scart, at least this is usually the case on Sony TV's