Stu,
Do you really need audio at your extended monitors? I don't, because of venue size, that my shows are located in... but in one, I do need more than a couple of monitors, which can be split between an HDMI and a RCA output... but I do have to run an extra RF Modulator between the RCA output and the TV. I used to have to do this prior to the addition of HDMI output, because of the age of the TV's in a couple of venues ( They haven't changed TV's in the 10 years I've been doing shows there.
), you might give the Modulator a try, mine do have audio output, but like I said, I don't use them!