Thanks for your comments, AG.
You might need an input impedance of 100k if the electret mic was just a bare electret element. However, the (cheap) electret mic capsules I've played with have had a built-in FET, as I understand it, providing a low output impedance (<1k??), so driving a 10k load doesn't attenuate their signal much.
Bearing in mind there's an internal FET and that the supply to the mic is only 4.5V by virtue of R4 and R5, do you still think 4k7 is too low for R6?
Since the mic is battery powered and at the 'line in' socket is not taking power from the PC (as it would at the 'mic in' socket) is the filter justified?