Hi, Sam. Right now, HDMI is only one-way. Meaning that your Onkyo is sending information to the TV through the HDMI cable, but the TV isn't sending information back through that cable. You'll need to make another connection to get sound from the TV.
Are you using an external tuner (such as a cable box, satellite receiver, or digital converter box)? If so, you may want to connect that directly to the Onkyo. Otherwise, you'll want to connect the TV to the Onkyo.
Does your TV have an optical audio output? I'm guessing that it does because it has an HDMI input, so it's a relatively new model. If it does, connect that to an optical audio input on the Onkyo. You can use any audio output from the TV, such as the analog audio outputs (red and white RCA jacks), but the optical is going to give a better quality sound.
BTW, did you mean TX-SR607 instead of TX-SA607? If so, then the optical audio inputs are assignable.