HDMI connected on Google Nextbook 8 when no HDMI connected
I actually had this very same issue. 

I contacted Nextbook and it was very hard to get in touch with them by phone or email. Finally I got an email back with some ideas, including doing a reset for the whole tablet. 

However, I believe what actually fixed it is that I bought a cheap HDMI to mini-HDMI cable on Amazon to plug into my TV (which works quite nicely) and once I plugged it in and out, that seemed to fix the problem of sound not working/screen rotating. I haven't seen the problem happen again, so I think there was just some slight connection problem that is now okay. I hope it works for you.