This has plagued me from day one but it's only since test driving other DAW's recently that I realized that Sonar's audio engine really does stand alone when it comes to crapping out while looping, especially in large projects. I'm working on something now in which most of the synths in the project have been bounced - there are six synths still active, nothing that should tax any DAW. High track count, and quite a few plugins, but again nothing outrageous, and my CPU meter is showing quite moderate activity. 
 
Despite this, Sonar chokes at the start of every loop iteration. It's almost like a needle sliding across a record. I think this is mainly with synth audio. 
 
Not only that, but I find that Sonar sometimes has trouble figuring out what audio is part of the loop and what audio isn't. It'll frequently play audio that is just outside the loop boundary upon each iteration. I've also seen this with MIDI - even when you have a note that is bang on quantized on the first beat of the measure after the loop boundary, Sonar will sometimes play it. This makes it a real PITA to work with a loop enabled (which I do a lot when mixing). There's this huge cringe moment every time the loop iterates as the audio craps out and stray notes are played. 
 
Furthermore, I'm also beginning to realize that having sidechain sends enabled makes Sonar crap out even more when looping. I have a loop enabled now that is performing horribly to begin with, but when I enable a send from the kick to a compressor's sidechain on a synth track, the audio engine stops at each iteration. Disable the send and it starts looping again. 
 
Current buffer size: 2048 samples. So it's not that. Has anyone happened upon any combination of settings that makes looping smoother in Sonar? 

Sharke, I really don't know what would solve your problems. I just finished a loop library called "Electronic Ear Candy" and while testing had 139 different looped layers up at one time, and was playing combinations of them as well as jumping between them with exclusive solo. The only time get tripped up with looping is if I make a selection with snap to zero-crossings turned on, then make a loop based on the selection.
 
However, mine are audio-only. You say you bounced the synths, but did you archive them? For diagnostic purposes, that would take them out of the picture entirely. Also the sidechaining might be an issue. 

I haven't seen this particular issue, but as Sharke mentioned previously I have seen issues with automation not being in sync with the audio during looping.  That definitely exists in the latest and last version of SONAR.

This is with synths - If I have a project that is 100% audio it loops fine. I have many synths in this project, but most of them are bounced to audio and of course I always turn them off in the rack after the audio is bounced. I have a handful of synths remaining. This isn't an overtaxed CPU thing because like I said, my CPU meter is showing very moderate activity. 
 
If it were a zero-crossing issue then I guess I could expect to hear a click upon each loop iteration, but it's a lot more serious than that. I've heard other people mention it in the past too.
 
The sidechain is definitely an issue, because it stops the audio engine outright when I enable it. 
 
I know Sonar doesn't have gapless audio, but I guess I was pleasantly surprised when I loaded a large demo project in Bitwig, fattened it up with a couple dozen more synth tracks, and was able to loop over and over seamlessly without so much as a click. Despite the lack of gapless audio, I feel Sonar should be able to handle this kind of looping without a hitch. 
 
I think there's some underlying problem with Sonar's loops. It's not just audio - I've documented a bug whereby Sonar will misalign automation envelopes while looping so that automation outside of the loop boundary is applied while the loop is playing. I reported that bug in 2014 and it's still listed as "open." 
 

Craig, when/where will this new loop library become available? I love Electronic Ear Candy!

Craig's comment about archiving would be good to test, since the audio engine is anticipating (most) things to be turned on/adjusted during playback. I am wondering if (for synths) the engine is scrubbing the settings and re-importing them each pass (since the end of the loop can be far different from the beginning). With MIDI note chasing enabled, this would just add more fuel to the fire since the audio engine could also be processing the synth to achieve the sound at the current play position (also potentially different for each loop). If you save as new project and archive those tracks, does it resolve?
 
Side-chaining is more of a mystery.

mrpippy2
Craig, when/where will this new loop library become available? I love Electronic Ear Candy!

Thanks for asking. It's been soft-launched on Reverb.com, with a couple audio examples. I just finished a 4-minute video and after it's posted, Reverb.com will do the "real" launch.

mettelus
Craig's comment about archiving would be good to test, since the audio engine is anticipating (most) things to be turned on/adjusted during playback. I am wondering if (for synths) the engine is scrubbing the settings and re-importing them each pass (since the end of the loop can be far different from the beginning). With MIDI note chasing enabled, this would just add more fuel to the fire since the audio engine could also be processing the synth to achieve the sound at the current play position (also potentially different for each loop). If you save as new project and archive those tracks, does it resolve?
 
Side-chaining is more of a mystery.

The synths are turned off in the rack. My workflow is that I bounce the audio, disconnect the track from the synth and then turn the synth off. Synths which are turned off are effectively "archived" though right, or is that something different? 

CoteRotie
I haven't seen this particular issue, but as Sharke mentioned previously I have seen issues with automation not being in sync with the audio during looping.  That definitely exists in the latest and last version of SONAR.

I think in general, Sonar has a problem with communicating the song position associated with looping. For example, I use Geist a lot and it has its own internal song sequencer which remains in sync with Sonar's. If however I toggle looping on or off during playback, Geist's sequencer goes out of sync with Sonar's, sometimes to the tune of 16 measures. I bet all these issues have a common root. 

You've identified bugs that plague you. You've been waiting years for them to be addressed. Sonar won't be improved past this point. You see where I'm heading James? Sonar is now my legacy DAW and I suggest you make it yours as well.