Recommendation - Setting up ONKYO - HT-R391

[s0619212]

I have got HT-R391 5.1 connected to my samsung TV. I have all the 5 speakers sitting on a table and the subwoofer on the groud. I have my Uverse cable box connected to the receiver and the output going to my TV, all these are HDMI. I can hear the sound through all 6 speakers, but i have to raise the volume really loud like at 50. I think i am missing some basic configuration. Unfortunately i cannot mount any speakers, for now i just want good sound . Can somone please recommend me what to do?