I just got caught out by the Windows 10 upgrade silencing Notation (I use version 2). I found the workaround here:
http://haskell.cs.yale.edu/euterpea/midi-on-windows/
but now see the fix is also mentioned on other threads.
Here are some more details.
1. Download
http://www.synthfont.com/SoundFonts/FluidR3_GM.sfArk
2. Download
http://www.synthfont.com/sfArk.zip and uncompress it (e.g. right-click and "extract all").
3. Open the sfArk folder and double-click sfArk.exe
http://www.synthfont.com/SoundFonts/FluidR3_GM.sfArk
4. Click "file" "open" and choose the FluidR3_GM.sfArk file you downloaded in step 1 (probably in your downloads folder). Click "open"
5. Click "file" "set target folder" and choose somewhere to store the FluidR3 _GM file (remember where this is).
6. Go to
http://coolsoft.altervista.org/en/virtualmidisynth
Scroll to the very bottom and click on
CoolSoft_VirtualMIDISynth_1.13.2.exe (don't worry if the numbers at the end differ from 1.13.2)
7. Double-click the file you just downloaded. If prompted, enter an administrator password. Install the program. Optionally, uncheck (untick) "VirtualMIDISynth needs soundfont files to work, open Coolsoft website ...".
8. You will now have the program "Configure Coolsoft VirtualMIDIsynth" open but you can open it again if needed from the Windows start menu or it may open automatically if needed when you start Notation.
9. Click the plus button at the top right, locate the FluidR3_GM file you downloaded in step 5 and click "open". Click "OK".
The links above came from
http://www.synthfont.com/soundfonts.html
Peter