NO sound in my SBL+SBR during DD5.1... what's wrong?

[Shoo]

I've recently formatted my PC, and after installing nVidia PureVideo Decoder, I managed to get DD5.1 and DTS displayed on my receiver. However, when I play a DVD (in this case, PotC - At Worlds End) with DD 5.1 sound, I get the "Dolby Digital" displayed on my RX-V663 receiver as well as LFE, L, R, SL and SR are displayed/lightened on the reciever.

Does this mean that there will be NO sound in my Surround Back-speakers when I watch the movie with DD5.1 sound? However, when I turn on "7ch Enhancer" on my receiver, there's sound in my SB-speakers as well, but the SBL+SBR lights on my receiver are still not available (which is normal since I am not playing a 7.1 movie) ... But why's there no sound in SB-speakers during DD5.1? How do I get the sound to work, or will it not work at all, hmm?

I do have a quite small and narrow room, as you can see on this picture:

(it's an old picture with just side-speakers, so don't mind about that)

So, how do I get this to work properly then..?

[Mark.Yudkin]

If you're saying you have a 7.1 setup and a 5.1 source, then yes, 7.1 - 5.1 = 2 unused rear speakers. Your receiver should have a setup option to specifiy what to do with the rears in such a case, the usual choice is "split" the side / rear at -3dB.

[Shoo]

I see.. but I cannot figure out how I "split" my rears to -3 dB on my receiver (Yamaha RX-V663) .. Is the splitting the same as going to 7ch Enhancer mode..?

Yes, a bit newbie on this, but I'd like to learn...