It all depends on how your system is set up. You should be able to use your TV's audio out connections. This way it's HDMI going in but RCA going out to your system. You can also hook up RCA cables from the blu-ray player to the system and still have HDMI hooked to the TV. I would recommend this way over the first idea.
To get sound from the home theater from your blu ray disc player you have to connect an optical cable from the player to the home theater. Hooking it up to the tv with HDMI only and using optical cable back to the home theater will only result in stereo sound (of course I have not done this, reason is the tv does not have the source for multi channel). Hope this will help you out.
Connect HDMI from Blu-ray player and cable box to TV for video then optical cable or composite cables ( red & white ) from them to Home Theater System for audio.