The first step in solving such problems is to understand what is happening. There is a tool to help you do that in MC.
Go to Help/System Info on the menu (or Service & Plugins/Reporter in the left navigation bar). Take a look at what is reported under the "Power" heading. It should tell you if MC is going to stop the PC going to sleep or not. Refresh the tool to watch changes over time.
If you are playing music, it should say; Playback (disable automatic sleep)
If playing video, it should also disable the screensaver and keep the monitor on.
Report back if these things aren't happening.
If that is all working as expected, you will need to create a MC log for someone from JRiver to look at.
Turn on logging: Help/Logging, Output to log file
Reset the log, same place
Hibernate your PC
Wake the PC, if MC crashes, save the log. Help/Logging, Report Problem
That will put a zip file onto your Desktop, which contains txt files of the current and previous log, and a system info file. Have a look in there to see if you can determine the problem. If not, ask back here if you can send it to someone to check.
