First of all, does the HDMI port the firestick is plugged into say DVI or Monitor or PC? If so, this port has no audio through the HDMI port.
Surround systems are hooked up via 1 of 2 methods.
Method 1 - TV is output only:
Equipment is plugged into surround sound receiver via HDMI, the surround receiver then outputs video via HDMI. For this setup you need to plug the firestick into the surround receiver.
Method 2 - Audio out to Receiver:
Equipment plugged into TV HDMI ports, then the TV outputs audio via either optical toslink connection or RCA (older white/red plug cable) to the audio input on receiver.
For this setup, the firestick should be plugged into the normal HDMI port on the TV, and the TV should already be configured to...