No pitch bend, I do touch nothing - and it happens, even if I disconnect the Midi keyboard.
That is really strange. Would you mind posting this as a Dorico file, so I could test it on my system?
There must be something truly untoward going on with your system configuration, Klaus. You have two computers, I seem to remember. Do you experience the problem on both computers, or only one?
Thanks Stephen,
it happens with any file and it happens randomly at different places. This time it was just the last four bars, the time before the movement started right away as quasi modern composition… If I do an audio export everything is fine.
Yes Daniel, something fishy is going on here.
I just fired up my MacBookPro and opened the same dorico file. Besides the fact that I had to change to Halion SE Elements, the playback was rock-solid, no problems.
… side note, the laptop had been offline for over 70 days - this happens when musicians don’t travel that much at the moment..
So I think we now have to look at what could be happening on your main system that is causing the pitch to change. My guess is that it will be the sample rate changing while Dorico is running. Can you think of any possible applications, utilities, or other processes that could be responsible for it?
speaking of humour, this extract actually reminds me of a sort of hurdy-gurdy fairground music in a Hitchcock film which is about to disintegrate followed by a murder… 
A few more bars would be interesting to see how things develop!
I’m afraid I cannot be of any practical help – Daniel’s most recent post pretty well sums things up .
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Daniel,
I did some testing yesterday and I have possibly (99%) found the culprit.
My M-Audio ekeys49 Midi keyboard is connected via a USB-cable. There is also a switch on the back, which probably just cuts the connection of the USB-cable.
I selected a point in my Dorico score and hit “P” for playback.
The usual thing happens, at some point one line / instrument of the score jumps up a halftone.
I hit “P” again - the playback stops.
I hit “P” again, the playback starts, now the same thing happens at a different spot.
All in all quite entertaining, as it never repeats itself.
Now - while still in playback - I switch off the USB-keyboard, the out of tune playback stays, as before.
But, if I hit “P” and “P” again to restart playback from the selected bar, playback is fine.
If I leave the keyboard switched off (without it been unplugged) there are no issues.
No out of tune playback surprises 
It is a very simple and straightforward Midi-keyboard, just the 49 keys, 2 wheels = controller für pitchbend und modulation, a volume control and two octave shift buttons.
I never use anything of those except the octave shift buttons. I guess (this was suggested in this forum) I will have to open the device and clean/“undust” it from the inside. Supposedly, if there are optical sensors involved, they’d rather prefer a dustfree environment…
[edit] in an earlier answer to Jesper I said “no pitch bend” - there might have been an invisible pitch bend command coming from that dusty keyboard…
How would this only involve one instrument and how would this be a different stave each time I repeated the playback ?
Might be a good idea with some Kontakt 60 or DeOxit or similar.
Jesper
Not a Dorico user, but I had my M-Audio Oxygen 61 play the exact same tricks on me. At first I thought it was just a case of unpaid musicians, but no, it was the pitch-bend wheel.
I opened up the keyboard and re-calibrated the pitch-bend wheel. (There is a screw that you need to tighten or loosen until the pitchbend reads 64 (on the keyboard’s display) in the middle, 127 up, and 0 down. You’ll need to have power connected for this to work, obviously.)
Just thought I’d share my misadventure, because after this… “operation” the keyboard works with no other problems. (I was very close to chucking it to the bin)
It is good to know that you are not alone 
The voices are made by SoundIron. But it’s not just their Olympus voices that I’ve found to be shocking flat. (Authentic choir… ) I bought their Requiem Light library, and that had exactly the same characteristics. I have brought it to their attention, and they said they would fix it in an update, but…
It might be worth mentioning to them that the Olympus sounds are flat too.