I think this is an issue with the Greek national digital channels encoding.
I am having the same issue. My Slingbox PRO-HD has a coaxial MPEG4 DVB-T decoder, but not an MPEG-2 decoder.
All DIGEA channels are in MPEG 4, except the Greek National (net, et1, et3) which have ONLY VIDEO in MPEG4, but audio in MPEG2 (still).
That's why you are having this issue. Try to get a digital mpeg4/mpeg3 digital receiver and connect it through composite/component..
hope this helps.
Btw, sling media should start supporting greece at some point, their lack of support is pathetic!