Musart - I assume English is not your native language, so it's not completely clear what you are asking. However, I'll give it a try...
You do not need to mixdown internally with Sonar first, before you export to wave. The simple process of exporting to a wave file does the mixdown as well.
There are two ways of getting a "mixed down track" (three ways if you rerecord everthing like
ViceVersa was doing in another thread

):
1. File > Export > RIFF Wave
2. Edit > Bounce to Tracks (make sure all tracks are highlighted first)
Either one of these processes will create a stereo mixdown of all your tracks. The difference is #1 will give you a standalone wave file that is
external to Sonar, whereas #2 will give you a new track
within Sonar with the mixdown on it.
So which method you use depends on what you are trying to do. If you want to burn to a CD, you need to use method 1. If you want to "master" the project, either method can work. If you want a backup copy of the mixdown housed in the project file, you would use method #2.