Can you confirm your version of Windows please 10 1511 is not a version I am familiar with, current released build is 10586.318 - you can check your version from running winver.
previous windows insider builds before retail version had lots of audio glitches/bugs in my experience where the solution was just that, restart and hope next update was around the corner. Sometimes the winupdate would get stuck, and need to log out and login again to MS account +clear windows update cache. Bit weird if he hasnt updated windows since retail release tho...
To be honest I am not really surprised and the reason I am asking about Windows versions is that applications tend to have massive problems on Insider Builds and so if this is Insider build specific it doesn't make sense to look into it. However if it happens on a normal build of Windows it might be worth looking into. Providing a log from a streaming/recording session after you've resumed might be a helpful first step, you can see how to upload a log in this post:
https://obsproject.com/forum/threads/please-post-a-log-with-your-issue-heres-how.23074/
My personal opinion from years of working on Windows is that I would not expect video and audio software to work very well after coming out of standby/hibernation.There is no reason for this that I know of it's just a gut feel.
Just restart OBS?
To your point on the Windows version that is the first time you have specified the version so great that's all cleared up.
It sounds like this is a driver issue which would explain why restarting OBS doesn't fix it but rebooting does. Drivers are susceptible to problems during sleep if they don't handle sleep states properly so I would make sure you are up to date on all audio and video drivers.
Last thing - please post a log it really really really helps :-)