Hmmm... you guys don't believe me, huh.
They are not backwards compatible. You can open Cakewalk files in Sonar, but not vice-versa.
First of all, in 1.3.1 the files extensions have been changed (.cwb and .cwp). PA9 wouldn't begin to know what to do with them.
Secondly, what is PA9 supposed to do with all the new features that have been built into Sonar?? Automatable FX, DXi's, slip editing, etc. etc.
Believe me - it can't be done. What you can do, Mr. FZ, is export your individual tracks as .wav files and have you friend import the .wav's into Cakewalk. He can then send you a bundle file (.bun) back, which you can open in Sonar.