I am using Sibelius 7.5.1, thewritescore template 7.0a, Mac OS Yosemite, VDL 2.5.5, MacBook Pro (13-inch, Late 2011), 2.4 GHz Intel Core i5, 16 GB 1333 MHz DDR3. 

I can open a project with out any problems. As I am working on a project for around 5-10 mins, the VDL sounds stop working?  The only way I have been able to remedy the problem is to save, quit and restart sibelius.  Any ideas on what I can do to fix this so I don't have to restart sibelius every 5 minutes?

Thanks
Kevin

Have you registered your copy of VDL? If it's unregistered and operating in demo mode, it will stop working after 15 minutes. If you open Service Center (the Native Instruments application that controls activations), you will be able to determine whether or not the library has been activated on that computer.

Thanks!!  I opened the service center and for some reason it wasn't registered or became unregistered.  Not sure how that happened.  I have had to reformat my hard drive a couple of times and upgraded the operating system to yosemite.  I will work with Native Instruments support to figure out why it is locked.  Weird!