I believe there is a Windows 7 compatibility mode for applications in Windows 8 - have you tried that?
(I am grasping there - if drivers are not available for Windows 8 you are most likely out of luck, which is why folks should really do their homework prior to upgrading to Windows 8).
Sorry for your difficulty, in any case, and I hope there is a solution for you. I do not recall seeing any such solution though, for anyone so far who has posted similar issues with non-supported interface drivers on Windows 8.
For whatever the worth, Windows 8 seems to work well with Sonar X2a, as long as the interface drivers are up to snuff and the system meets the stated requirements for Sonar, as found on the Cakewalk site.
Bob Bone