I would be interested to know what your signal flow is. Where are your external synths connected? (direct to the audio interface is not the place)

For me I have 6 external instruments and they all connect to a analog mixer. Synths have analog outputs so you need to connect them to an analog mixer.  I also use a digital mixer and have the analog synth mixer connected to the digital mixer. (I could connect them direct to the digital mixer analog inputs too and I did but I am using those inputs for other stuff, mics mainly)

That way I am always monitoring the outputs of the synths directly and not through any software. That is the only way to do it really.

If I don't want to turn any external synths to audio I simply let them play. I do a mix of those on the analog mixer. My DAW is always recording the output of the digital mixer which has the DAW outs of course connected there (digitally) and anything else that might be coming in. I never get any sync issues that way.

Turning external synths to audio is easy as I can direct any of the inputs on the analog desk straight to the audio interface (which is in my case the digital mixer ) And yes you just do it one by one in real time. 

The analog mixer works great because it has some real advantages. One is the Mic Pre gains can be matched to the synth sources so well. Some synths can be very loud and others quite soft. That mixer also has HPF and great EQ so any external synths can be altered a bit (or a lot) on the way into the DAW. There is a lot you can do there before the synth is even recorded. That mixer also has a nice effects processor built in and it sounds excellent.

In answer to you both (typed a long one first and it was eaten by the forum somehow): I've been doing it for ages in Cubase, it's very easy and convenient. External synths' output plugged to soundcard's input and to its own audio track in DAW. Live monitoring is always on in such tracks and when live mixdown to file is done its input is mixed in together by the DAW. That also allows to put plugins on them, like console emulation, EQ and noise gate for example, doing a final mix, and run it through the mastering chain - all in the DAW. Mixing it externally on analog mixer is not a good option for me, because then I can't do mastering right on the main bus, which is very convenient. And no recall - so I can't get back to it later and get exactly the same mix. Also, I don't have a high-end analog mixer and there is no point doing final mixing outboard otherwise.

Hi Roman yes I suspected you may have been doing something like that and the input monitoring in Cubase was obviously such that you could do it too which is interesting. 

Just out of interest I don't do the final mixes using the analog mixer either. Once I am happy with the midi the way it is, I do turn all that stuff into audio and have all those parts sitting inside my DAW too on their own tracks and do complete ITB mixes from there. I basically use the external analog mixer just for monitoring while I am building up the arrangement.

Anyway this is not helping too much in your situation. Perhaps a small cheap analog mixer might work where you can have the synth connected to there and the output of your DAW connected there too just for monitoring purposes. It may be a simple workaround while you are using Sonar. I have always felt that while you are working up your music that has always been the best way for me and there are usually no sync issues to speak of.

Sonar's audio/midi sync may not be the issue and it sounds like it is OK actually. What you may be hearing is the input monitoring latency due to the heavy audio processing that is going on your masterbuss. The good thing about any external mixers is they avoid all that.

I've been doing this in Sonar before, not just Cubase (just real-time mix with "include live inputs" gives you the same mixdown), and it was working fine until I started doing mastering ITB as well. When I was doing mastering on the final mix file later, delay was not noticeable (but it was there I guess). I do have a decent analog mixer, so that's not the problem, also my Tascam FW-1082 audio interface can monitor and route inputs separately. I just want to do it all ITB on the fly. It's easy, convenient and is technically possible. But because this stupid bug it's not workable in Sonar with mastering or other delay-compensation-inducing plugins.