You would need to connect the ARC HDMI connector on the TV to an ARC capable HDMI connector on the stereo. If the stereo HDMI is not ARC compatible, then you'll have to connect HDMI to HDMI then use a optical audio cable or even the old school RCA (Red and White cables).
Also, on my old stereo you had to set the output on the stereo and the input on the TV. So if the HDMI was plugged into HDMI 1, the the TV would need to set to HDMI 1. If the HDMI was in HDMI 2 on the Stereo, the stereo also had to be set to HDMI 2. (AM, FM, S-Video, HDMI 1, etc).
I'd first make sure that the stereo is set to pull data from the correct HDMI port first.