Use the optical or coaxial digital output on the Blu-ray player. Make sure the Blu-ray player is set to output over an SPDIF connection (coax/optical) instead of HDMI. Also make sure it's set to bitstream.
Depending on which Sony Blu-ray player you have you might be able to utilize 5.1 analog output. The new BDP-S550 supports this feature, the lower model BDP-S350 does not. I am not as knowledgeable on the older Sony manufactured Blu-ray players.