If I need to create mp3 files, I always do the conversion outside of SONAR.
And I've never got better results than using my ancient, but trusty, copy of Pinnacle/Steinberg
MyMP3Pro (v.4).
It gives you the choice between using either a Lame encoder or a Fraunhofer encoder, and I have to say, the Fraunhofer does a
much better job than the Lame. At 320kbps, it's very difficult to tell the difference between the mp3 and the original wav file.
I think they gave up making it after version 5, but if anyone comes across a copy sitting around, it's well worth getting hold of.
Now, if anyone knows how to 'extract' the Fraunhofer encoder and use it inside of SONAR, I'd be really grateful!