Having got one for Christmas I have the same problem. I can't find one either!
I think the only way will be to use a Maplins speaker multi switch box I have and connect a pair of lines from it to a female headphone socket. Not the best solution but Sony seem to have made a bit of an omisson here!
On a different topic I bought this CMT- MX750 from John Lewis and was told by their tech guys that an ex demo model Sony NAS C5 wireless box I bought online a few weeks ago would recieve audio from the CMT-MX750 in it's party mode. The manual implies that I should be able to "party". Having failed to get it to work I rang Sony and was told that despite being able to use all the other functions on the 750 unit quite happily, streaming from my PC etc (works brilliantly) I cannot use the 750 to send the audio to the Sony NAS C5 wireless receiver in my kitchen. Rubbish really - I am waiting for their tech department to call me with the units that are compatible -if any!!
Not very happy about these missing functions but I do really love the unit as a whole. It does ALMOST everything I want!
