Soft synth and hardware synths not in sync

How much of an error are you talking about in milliseconds or samples?

You might need to dial in your record latency compensation (Preferences > Audio > Sync and Caching > Record Latency Adjustment) with a Manual Offset (not to be confused with Timing Offset on the same page -see below). But unless it's disabled completely and you're running a relatively large ASIO buffer, MIDI transmission and synth response delays would usually dwarf audio latency compensation errors, and neither would be audibly obvious in most cases.


Start by Googling the free CEntrance Latency Tester (assuming you're running ASIO Drivers), and use that to get the true round-trip latency for an analog loopback. The Manual Offset is then the difference between what CEntrance detects and what SONAR reports for the the Total Round Trip time on the Audio Drivers page of preferences. If SONAR thinks it's less than it really is, you need a positive Manual Offset added to the automatic compensation. 

Once you've got that set, you're ready to start doing some MIDI loopback/response time tests, and see where the delays are with your hardware synths, knowing that recorded audio is perfectly in sync with audio playback (and SONAR's audio metronome). The Timing Offset setting mentioned earlier can be used to adjust the relationship between audio and MIDI playback and recording, but it works differently with soft synths and hardware synths, and can be more trouble than it's worth, depending on how your system and projects are configured, and how bad the error is.

I tend not to worry about sync errors with hardware synths that are typically on the order of ±3ms so long as they're fixed offsets.

I, too, use a mix of soft and hardware synths. The soft synths are always right on, but I have to set the manual offset that Brundlefly mentioned for external synths to line up properly. 

Put a single quantized MIDI note at the front of the MIDI track so you know exactly where the the recorded audio should land, nudge the audio until it lines up and calculate how many samples off it is. Put that number into the manual offset. You'll only have to do this once.