Or try this. this working in my case:
Mark clip or section in audioclip.
Right click: process effect -> audio effects -> RX6 Connect
In dialog from RX connect click "OK"
do your work in RX 6 editor
In RX6 editor: "send back"
return to SONAR --->
In Sonar: Right click: process effect -> audio effects -> RX6 Connect
In dialog from RX connect click "OK"
Ready !!
Note:
Don't click "repair". That has never worked for me.
And since RX6 the clip is split if you edit a section of the clip.
iZotope RX within the Utilieties menu has never worked for me.