Hello,
leave the Blu-ray connected to the TV. Now run a separate connection from the "audio out" of the Blu-ray to "audio in" at the back of your home theater. That is, you will connect the blu-ray to the theater for sound.
Get a RCA cord (Red and white), connect it to Audio Out, at the back of your blu-ray. Then connect the other end to "Aux in" or "CD in" or "video in", depending on the input at the back of the theater, but it has to be "Audio in" (Red and White/ L&R).
After the connection, select the input on your theater. Take for instance, if you connect the input on "Aux", press "Aux" on the theater, and sound will come out through that channel.
This is what you are going to do, in order to get sound from the theater system. So, the HDMI between the player and the TV will bring display.
I hope the above makes sense to you or hope you understand the above.
In case you don't know what a RCA cord looks like, below is a image that will help you out;
Good luck.