Hi! I'm running Linux Mint 17 in dual boot on my G751JY and I had a similar problem - there was no sound coming from the notebook at all. I later found out that it had something to do with Windows 8.1 not shutting down properly when I reboot so I have to shut down Windows and then boot Linux to have any sound at all instead of simply restarting into the other OS. If you're in a dual boot configuration, this might be the solution for you as well. It's kind of annoying to shut down and then restart but with the notebook's fast boot I really don't mind that much.
Another issue I had was with the headphones - in Linux they wouldn't be recognised at all. Strangely I found a solution - plugged them into the third audio jack which was advertised as line-in but is apparently line-out. That worked like a charm. Alternatively, I remember reading about a fix which would allow you to use the standard headphone jack but it required editing some settings files and honestly I couldn't be bothered 😄 Nonetheless, if you encounter a similar problem, I hope what I've mentioned helps at least as a temporary solution