I have opened a trouble report with Cakewalk due to the Dorchester (Platinum in my case) update now wanting to set Sonar as the default player for .mid files.  This has never been a problem before (I've had Sonar since V7) and has only happened with the Dorchester update.  
 
I have a dual boot system with Win 7/8.1.  I installed the Dorchester update in my Win 8.1 installation to see if it was an OS specific issue and it is not, its a Sonar issue.
 
I have the "Win Patrol" program that monitors all system activites and it is reporting the attempted change.
 
Well, I fixed it.  I rolled back to the previous version and then rebooted the PC.  No .mid request from Sonar.  I then reinstalled the Dorchester update.  NO .mid requests.   However, I had to do this on both Win 7 and Win 8.1

I'm not seeing it here.  I have left VLC as the default for .mid files for forever.  When I saw your post, I verified thru CP.  I found .midi unassociated, so I set it to VLC, same as .mid
 
Came back and fired up S-Plat with no problems.
 
Is it possible that S-Plat is just checking and WinPatrol is erroneously reporting it as a change attempt?
 
What happens if you disable WinPatrol?  Does S-Plat actually change the association, verifiable thru CP?
 
 

Same here, Dorchester did not change my default association. I don't run WinPatrol.

I did a test.  I let Sonar change the .mid.  I stopped getting the notices.  I then went back and changed it back to Windows Media Player and then I started getting the notices again. 
 

One difference is Bob and I are not using WMP as the default MIDI player. I don't use VLC but do use 3rd party player. I believe the player is being picked up from HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.mid\UserChoice key Progid

Well, I fixed it.  I rolled back to the previous version and then rebooted the PC.  No .mid request from Sonar.  I then reinstalled the Dorchester update.  NO .mid requests.   However, I had to do this on both Win 7 and Win 8.1

glad you got it sorted out, tho I'm not sure why that would have solved it.

Beagle
glad you got it sorted out, tho I'm not sure why that would have solved it.

Agreed, at this stage... A full virus scan followed by a full scan (with rootkit) with Malwarebytes is essential.

My hunch though is that the issue is to do with Win Patrol (or another scanning utility) locking something at the wrong moment.

Its not malware, I run a full Malwarebytes (paid version) scan daily.  I also ran a Antivirus scan and nothing showed up.
 
Being a computer tech, I went deep into the OS, including the registry to look and found nothing there. Win Patrol is a favorite of many techies that I deal with on computer forums. 
 
It had to be something with Sonar.  I was not having the issue until the Dorchester update.  Falling back to the previous version and the problem went away.  Reinstalling the Dorchester and it hasn't done it since (and I've rebooted several times, and run Sonar several times).  Win Patrol runs at startup and is a real-time monitor.

Why would a removal and then reinstall fix it then?
Never mind :)