I've done some searching, this is a perennial problem-o.  I can't get it to work...I have 12 megs ram and a dual core AMD processor, MSI motherboard.  I start SPLAT, start new project and then *insert-->rewire--> sibelius 8*and crash!  I have to cntrl alt delete and end process to carry on.  I'm sure I'm missing some details....  

So after some trial and error, I got it to work by unchecking the low latency option in sibelius.  It probably has something to do with my Tascam US-1200 drivers?  Anyways, audio engine isn't crashing but hard disk usage soars. SPLAT and Sibelius 8 are both so new something'll probably get fixed.
 
Cheers!