So I've been running Sibelius 6 on a mac forever.  Downgraded from 7 immediately after it came out as I hated it.  Did not update my operating system also forever as I was scared things wouldn't work.  Eventually all of my internet browsers wouldn't work on the old operating system so I was forced to upgrade.  This made Sibelius no longer work and forced me into getting 8 a little over a month ago.  I have had a number of problems with sounds not working, and figured I'd have to redo all scores of past work clients would want to adapt.  However, I am also seeing this likely be because I was still running Kontakt 4.  Only in AU as VST was not an option but I read that Kontakt 4 was not compatible anyway.  I downloaded Kontakt 5 today.  Tried resetting all my playback devices and then the program crashed on me.  It continues crashing every time I open a score.  If I open just the program and try opening a score then it'll crash again.  If I open the program, start a new score it'll open so I changed the default to general midi when opening.  Then I tried adding one instance of kontakt 5.  As soon as I say ok the program crashes.  I have tried this a number of times with no success.  Contacted Avid and created a ticket, but usually the people on here seem more than capable of solving any problem I encounter and faster than Avid.  So, any suggestions?  Thanks in advance!

Mac OS 10.11.6
Sibelius 8.4.2

Also utilize the following
VDL 2.5
COMB2
Kontakt 5
Absynth 5
FM 8
Battery 3

This may not work, but it is worth a try. I see you have indicated you have been using AU instead of VST. But, maybe the default Playback Device was set to VST. Take a look at this Knowledgebase article and see if it helps.

https://www.tapspace.com/hesksupport/knowledgebase.php?article=126

That was helpful.  The program is no longer crashing on me, but now it is not playing the correct sounds.  Its all pianos for the drums.

Attach a screen shot of your Playback Configuration.

Thank you for responding and sorry for the delay.  I have been working around what I could as much as possible.  This is what I currently have.  I have gotten drum sounds integrated.  Though, the biggest issue I now have is with suspended cymbals as well as cymbal sounds in combos.  Don't sound like what they did before and the rolls are all now crazy long regardless of the keymap or midi input..

Are you sure everything in the Mixer is pointing to the correct playback device?

I am not sure what was wrong.  Over the holiday break I reinstalled a lot of the devices and setup my template off the version I bought again.  So far I am getting sounds that I am intending on what I am writing.  I will let you know if anything gets wonky again.  Thank you for the assistance.