As I said, the bit depth of the new file depends on the Bit Depth setting you choose in the Import dialog, or whatever you have set in Preferences > File > Audio Data if the file is dragged in. One option (the default...?) is "Original", meaning the bit depth will not be changed. For MP3s I presume this means 16-bit.

Thanks for all the replies.  I was indeed talking about importing wav files of different sample rates into the same project.   I did a test with 2 projects, one at 44.1 and one at 48, and imported samples at 44.1 and 48 into each.  They played back exactly the same in both, so as was stated above, seems Sonar does its job, without the user having to think.
 
Is there any noticeable difference in quality between converting a 44.1 sample up to 48, and converting a 48 sample down to 44.1 ...?  None that I could hear in my test but, logically anything ..?

SONAR's SRC is very high quality so you won't hear any artifacts converting between different sample rates.

bitflipper
It's a greatly under-appreciated feature that keeps things simple because under most circumstances we're entirely relieved from having to think about such things.

+1 to this comment. I have only used CW products so have been jaded by this "under-the-hood" nicety. It was not until a couple months ago that I first witnessed the issues this can create in some other programs, and it was at that point when I realized all the years I have taken this for granted.

That's one of the things that really sets Sonar apart and why its such a joy to work in, because you can actually 'work' without getting stuck in the 'tech' zone. Having to work in other DAW's for project compatibility has really made me appreciate how simple Sonar makes things that other DAW's don't.

s there any noticeable difference in quality between converting a 44.1 sample up to 48, and converting a 48 sample down to 44.1 ...? None that I could hear in my test but, logically anything ..?

 
No.. If there was, Sonar would be out of business.

bitflipper
Changing the sample rate of a project is a much bigger deal than on-the-fly SRC when you import a file.

Going back to this comment --- if I have a entire project that is 44.1 and I want to change it 48 - actually, I don't figure out how to do that - is it possible?  Or, is it that should when exporting as a WAV file, export it at 48?

How to change a project from one sample rate to another http://forum.cakewalk.com/FindPost/2934357

Thanks for that - seems quite a lot of work for a large existing project.  I could export a 44.1 project as a 48 WAV file, so converting all in one go to a 48000 file - not sure if there is any lowering of quality of I do that from 44 to 48 though?

the src shootout: http://src.infinitewave.ca/ although the latest version of sonar tested is 8.5 (has the engine changed since then?)