I noticed a lot of people complaining about not getting sound from tab-casting, youtube, netflix, or what-have-you. And when I first hooked up my ChromeCast, I had the exact same problem and I fixed it very easily. I know that a lot of people, once they fix their issue, don't bother to report back and explain what they did, thus leaving the community with un-answered questions. Now, that being said, I'm not saying my fix will fix your issue, but it fixed mine and it should be at least considered "noteworthy".
First, my setup:
I have a 50" Vizio 1080p HDTV that I bought from Sams Club in late 2012 around Christmas time.
Model number: E500I-A1
The router I'm using is a Netgear N600.
This is what happened:
I plugged it in, set it up via the software on my Late 2013 27" iMac and everything worked flawlessly (as far as setup goes). The video was 1080p, streaming was amazing, delay between pause/play to TV was less than 1 second. But there was no audio.
I tried various steps, troubleshooting, and changing ports, resetting up the device, etc, all to no avail. So what solved it?
Turning off the TV, and turning it back on.
I pressed play on a video that would run longer than a few minutes, and rebooted my TV, when it came back on, I heard the sound. The video was still outstanding. So maybe that is the problem with a lot of these reports. I just wanted to report how I fixed my issue.