<p>Don't you get a free download of Sonar Producer 8.5 if you bought 8.0 version AFTER 2009?</p>

In the past Cakewalk had offers to "buy now and get the next version free." They would happen a month or so before the new release. The deadline for such offers was around the release of the next version. There was never a situation where such an offer lacked a deadline. I do not recall if there was such an offer for 8.0 or the details but would not be surprised if one existed. Version 8.5 was released in Sep 2009.
 
I would look in the knowledge base for updates to versions X3 and older (including SONAR 8). My guess is the 8.0 updates are still there. AFAIK all the support pages which held the updates were all moved into the knowledge base a few months ago.

Unfortunately, there is no way to obtain a legal copy of 8.5. That's a shame, as it was a very good release and fixed a lot of issues from 8.3. It'd be a nice public service if CW offered it for sale, even on an as-is no-support basis. But that's not their business model; they'd prefer everyone to stay up-to-date (which, as a software developer myself, I can totally agree with - it greatly simplifies support).
 
Your previous purchase does, however, entitle you to a discounted price for the current version. I think it'll run you about $200, and that's with free updates forever. That's the route I'd recommend. You can buy SONAR direct from Cakewalk or from the major online retailers such as Sweetwater and JRRShop - same price regardless of where you get it.

It looks like Sonar 8.5 was the first version that claimed to be compatible with Win 7. It is not at all unusual for application vendors to limit support to a certain version of the OS, but there are remarkably few complaints on this forum history about people being unable to run 8.3 on Win 7, which suggests that some people were able to do so. One thing that does keep showing up in the forum history is that you need to run the installation "as administrator."
 
I do not want to insult your intelligence, but many people do not know that there is a difference between running a program "as administrator" and running a program from an account that is a member of the "administrators group." In XP it was common to run everything from an administrator group account, which gave you pretty much administrator privileges. Since Vista User Account Control (UAC), has been introduced, and an account that is a member of the administrators group does not have the full privileges needed for all operations. In particular access  to locations and files (permissions and ownership) is not automatically granted to administrator accounts. If you have not already done so, I suggest you uninstall your Sonar 8 installation and reinstall it by right clicking the setup.exe file and choosing "run as administrator" from the dropdown menu. 
 
http://www.digitalcitizen.life/how-run-programs-administrator-windows-7
 

@Slartabartfast - Yes, I installed program as ADMIN and I'm running in Compatibility mode for Win 7.
 
See POSTS #7 & #8
 
In addition, I just ran a 2nd test to confirm.
 
Program refuses to open previously save mixes saying they are not compatible with current version.
 
My program is still not accepting reg code and says I have 15 days left.
 
 
 
 

If I understand you correctly, you previously had this version of Sonar (8.3) running in Win 7 successfully?
 
If that's the case, then something has gotten corrupted, and you'll need to do (at least) a clean uninstall and reinstall of 8.3.
 
And by "clean", I mean you uninstall the program, delete any leftover Sonar folders; then, in the registry editor, delete any remnants of Sonar.  Reboot, then install Sonar with latest patch.  (Do all of these with "Run as Administrator").
 
If the idea of possibly upgrading to Platinum (and the lifetime updates!) tickles your fancy... download the SPlat demo and start by  importing your old projects.

If it is telling you you have a limited time left, that probably means that it has not accepted your confirmation code and you are still in demo mode. As I recall even back to SONAR 8 this was a two step process. You would put your serial number in, and Cakewalk would send you a code back that would unlock the program permanently. I can't recall if saving files was blocked while in demo mode then. I am pretty sure it is now. If that is the problem, then you will need to access your Cakewalk online account to get the proper credentials to unlock the program. Although Cakewalk may be unwilling to help with SONAR 8 problems, I expect they would feel obligated to helping you get into your account if an invalid registration code or serial number is the problem. No one else will be able to help you get those codes legitimately. 
 
https://www.cakewalk.com/...-product-serial-number

To the OP,
 
Did you previously have Sonar 8.3 installed on Win 7 and running successfully?  Do you have a serial number, and if so, is it the one you used previously?
 
If the answer to both of those is "yes", then see my earlier recommendation. 

Yes to both
 
I'm very wary about messing with Regedit....

I guess one question is if 8.3.1 was working under Win7, what changed so it no longer works?

Is this a previously stable and working installation that started acting up or a fresh installation on a new PC or....?