What's important to know is "actual" latency vs. reported latency. The big component of actual latency that's usually missing from the latency that the interface driver reports to the DAW software at any particular buffer setting is the latency of the bus hardware/firmware that connects the interface to the PC. With PCI(e) interfaces, the bus latency is usually quite low, but with external interfaces connected by USB/Firewire, etc., it can get to be a significant part of the total latency when the buffer size is low.
Good drivers will take this into account along with A/D/A conversion latency and report a pretty accurate number to the DAW, but some do not. One reason it's important is because the DAW is going to compensate for input latency by shifting recordings earlier by the reported amount after the fact. If that value isn't accurate, playback can be out of sync with soft synths, imported audio, and SONAR's audio metronome.
If you know the actual, measured latency, you can subtract the reported value and set the difference as a Manual Offset in Preferences > Audio > Sync and Caching to do additional compensation of this unreported latency to get perfect sync.
Check out the free CEntrance Latency Tester to ping a physical loopback patch between an output and an input on your interface to measure the actual latency.
http://centrance.com/products/ltu/.
The old-school method is to re-record an audio track by loopback, let SONAR compensate per the reported latency, and count how many additional samples you have to nudge it back to align with the source track while zoomed in far enough to see detailed waveforms.
post edited by brundlefly - 2015/11/10 20:28:22