It'll take some time and patience, but yes, the files can be salvaged.
The problem is that even though the data was sampled at 44.1, the wave files were stamped as being 48 when SONAR wrote them to disk. What you'll have to do is alter the "48" in the file headers, changing it to "44.1" so that SONAR can correctly interpret them.
In the past I've written instructions for doing this using a hex editor. I can tell you how to do that if you like, but it's safer (although just as laborious) to use a capable audio editor. Of course, no matter which course of action you take, you should be working on a copy of the project. Save the original files until you're sure all's well.
Adobe Audition / Cool Edit Pro, Sound Forge and WaveLab can all be used to effect this repair, but if you don't have any of those you can download the free
Audacity. You can find instructions for each of those editors
here.
(NOTE: the most-respected and widely-used software for sample rate conversion, especially for large numbers of files, is Voxengo's
R8brain. I recommend adding it to your toolkit. Unfortunately, it cannot perform this particular operation. At least, the free version can't.)