Just for the heck of it, I decided to go through my Sonar preferences today, and I found a few settings which were different from my normal settings.  Settings which I don't normally use, were active.
 
Recording mode - Was set on Sound On Sound.  I normally have it set to Comping
Clock Source - Was set to Audio.  I normally have this set to Internal.
Record Count In - Was set back to 0.  I normally keep it at 2
 
Also, I hadn't noticed it, because the themes are pretty similar, but my current theme had changed from Mercsten back to Tungsten.
 
Not sure why this happened, just thought I'd put it out there, since these changes seemed to happen without my input.

i'm pretty sure that after the last update (2016.11) the theme i use, tiffany blue with blue meters was changed back to the default tungsten, but no biggie to just change it back.

davec69
Recording mode - Was set on Sound On Sound.  I normally have it set to Comping
Clock Source - Was set to Audio.  I normally have this set to Internal.
Record Count In - Was set back to 0.  I normally keep it at 2

I believe all of these settings are project-specific so they'll vary depending on what you have open, and they may default to those settings if you have no project open.
 
Clock source must be Audio if there's any audio in the project, including the metronome and soft synths. 'Internal' is applicable only to pure MIDI projects, and SONAR will automatically switch to the clock to Audio if that's not the case.

The default for new projects is sound on sound. I find this annoying as out of all 3 settings this would be the most unpopular setting. I wish once set that this setting was a global and would never change. 
I always have to remember to change it to overwrite. Why on earth would I want a bad take mixed in with a new one??
I don't use it much but at least Comping makes more sense as a default. 
And sound on sound?? who here actually uses it and please explain to me other than having a cacophony* of layered guitar parts,, or even worse bass??? what is it's purpose?  I seem to remember the term from early tape decks where this was your only choice for adding new parts. 
 
 
 
ca·coph·o·ny
kəˈkäfənē/
noun
 

a harsh, discordant mixture of sounds.
"a cacophony of deafening alarm bells"
  

Change it to Comping in your default template, then save the default template. Mine always comes up in Comping mode.

Cactus Music
The default for new projects is sound on sound.

not the case here, you need to check the template you are calling up for the new project (the one marked with a "star" symbol in "new from template" on the start screen), which is also the default when startinig projects from the file menu
 

I find Sound on Sound very useful when building up Midi drum parts on the fly.

Bristol_Jonesey
I find Sound on Sound very useful when building up Midi drum parts on the fly.

Yes, recording MIDI drum parts from a keyboard is the one case where I commonly use sound on Sound also. I think that was mentioned in the other thread about the record mode defaulting to Comp Recording.
 
I also frequently record an improvised melody as a layered take on the main MIDI piano track of a new piece and then go back later and re-record it using some other synth, or even just polish up the rough take and use it on another track.

Thanks everyone, for clearing this up.  In summary:
 
*  Some settings are project specific.
*  Recording mode - Has been changed to default to Sound On Sound instead of Comping.
*  Change Recording Mode in your default template and save to set a default.
*  Clock Source - Audio is the correct clock setting if the project contains audio tracks.
*  Many users prefer Sound On Sound mode when recording drum parts.