FirstPullUp
In response to tlw above, neither Win10 installation is able to find the ASIO drivers, and so they're using (I think) MME - I'm not a driver expert. And I get a funny audio profile saying nothing below 88K (I think) is available.
 
 

that is probably your problem right there. MME is the worst.
If you were on platinum you could try using (on windows 10) the new WASAPI driver mode. you didn't give enough info about your version. which Sonar Producer ? version ?
 
try playing with your buffer rate in your preferences/drivers.
Either way you look at it, if you want good results an audio interface is the way to go.
if your project is heavy, some plug ins, tracks, its going to bog down using MME. that's the windows driver.
a cheap used Focusrite soundcard is about under 100 bucks.

MME doth indeed suck, but Win10 seems to have stripped out the ASIO drivers in an update a few months ago, from what I've been able to learn from Microsoft forums. So I'm stuck (at least on the machine I want to use, big ol' Dell tower, which doesn't have a Dell ASIO driver - I haven't checked for my laptop yet). 
 
The sound goes bad with a single audio track. (The mix has fifteen, but I've tried it with a single track.) I'll play with buffer rates and such. Given that I'm using this only for mixing, I'd rather not invest in a sound card, but I suppose I might. I wonder if an older sound card would work? I have a few lying around.

ASIO drivers work only with ASIO-compatible hardware, not Windows on-board audio. It really is worth getting an audio interface with ASIO support. Otherwise it's like using a Nikon with a scratched lens that's smeared with pizza sauce. It will take a picture, but...

Neither the interface nor its drivers come into play when importing files. They are 44.1 files. Assuming the project sample rate is the same the import should essentially be a file copy operation with no opportunity for distortion. That's why I suggested importing "known good" files as a test.

bitflipper
Neither the interface nor its drivers come into play when importing files. They are 44.1 files. Assuming the project sample rate is the same the import should essentially be a file copy operation with no opportunity for distortion. That's why I suggested importing "known good" files as a test.

while I agree with what you are saying, if sonar is using MME with a bad buffer rate, clicks and pops are inevitable.
 

I used MME successfully for a year or so before buying my first interface.
 
I think I got something like this...
http://www.musiciansfrien...p;kwid=productads-adid^156727059247-device^c-plaid^140859209541-sku^250006000000000@ADL4MF-adType^PLA
 
It was fun because it came with a lot of Line6 Amp Simulations and was also small and easy to throw in the laptop bag. I eventually gave it to a cousin.
 
I was able to mix with MME, but I couldn't record reliably until I bought an ASIO compatible interface.
 
I'm posting mostly to let you know that I was able to use MME for my first year or so. My experience was substantially improved by the little interface.