A gate is very final in it's operation. If a sound does not meet the threshold it does not get through simple as that.
A downward expander is the tool you need. Now you will have to find it in Sonar. As I am not a Sonar user I am not able to point you directly to it.
The way a downward expander works is as the signal falls below the threshold, the further away it gets from the threshold (ie lower) the more it drops the gain down to silence the areas in between any singing or talking and it does work very well. The good thing about it is even if a sound comes through and it is still under the threshold then the expander will still open up and let it through so you don't loose important low level detail.
It may be a special plugin or a another plug with a different ratio settings. I am pretty sure Sonar has one though.
You need to spend time with a downward expander in order to get it working nicely and more importantly in a very transparent way. Once you master setting it up you can also get into using it with the vocal mike on the way IN to your DAW.
(if you have a hardware one outside your interface that is as I do)