This sounds crazy, but did you try reversing the HDMI cable? That sometimes works.
Also ran across this from another site:
"The same problem exists on other HDMI attached TV boxes, whether it be DISH, DirecTV or most IPTV boxes.
It has to do with the audio signals on the HDMI ‘ports’ getting their knickers in a twist when that HDMI port is selected. This, alas, happens when you switch from one HDMI port to another — in this case when you switch to the Roku HDMI port.
The fix is actually a work around.
Don’t switch directly from another HDMI port to the Roku port. First switch to a non-HDMI port. Wait for 3 seconds. One Mississippi. Two Mississippi. Three Mississippi.
Now switch to the Roku HDMI port. Bob will be your uncle.
On my 47″ Philips TV the Roku is on HDMI port 3. The DirecTV Genie box is on HDMI I and I have a DVD player on HDMI 2.
I no longer go straight from HDMI 1 (DirecTV) to HDMI 3 (Roku).
I first go to the ‘TV’ (RCA jack) port on my TV. Wait 3 seconds and switch to HDMI. Sound works first time.
I have known about this HDMI problem since 2011. Had it with DISH and then with DirecTV. "