Use a VU meter. Just out of interest does Sonar have clip gain as in Pro Tools and Studio One. If not that is a major major oversight. Once you have it and work with it you will never go back.
You don't need any special tools either as per
Dave's suggestion. You just drag the amplitude of clips up or down as per PT and Studio One and simply match the rms part of the signal by eye and that's it. Fast and effective.
Inside Sonar I would get a VU meter plugin, calibrate the system and then simply watch where the meters are reading for your various clips etc. Then simply adjust the gain of the various clips so the VU meter is showing the same for all of them and you will end up with them all sounding very close in terms of volume.