Hi Alan, You might try entering the C1 keyswitch (which in Sonar is C3) to activate the default mode. If this doesn't work for you, post again please.
By the by, I have a lot more luck with the Strad and Gofriller in the Kontakt Player, which they were programmed for, and not the full Kontakt.
Good luck and have fun! John
You said you've tried various keyswitches - are you sure you've engaged C1 as John "Big Ears" suggested? I don't know why the Cello should be loading in anything but its default C1 condition - but that sounds like that's what is happening. And of course you know you need to have some cc11 volume control immediately in the project, or the Cello will sound very odd.
Hi Alan, When you have the cello loaded in the Kontakt Player and inserted in your project in Sonar, do you have the keyswitch for the default sound shown in the piano roll view? If not, maybe try putting it in at the very beginning of the first measure (remember it is not C0 in Sonar, I think is is up a couple of octaves, maybe C2, I don't have my DAW fired up right now, but if you play with the keyboard on the piano roll view left side you can find where the various keyswitches are in Sonar) and also maybe draw in some CC#11 data to give the expression control a wake up. With the expression control at 0, the tone of the Gofriller is really flat and that may be that's what you are hearing. Until you raise the cc#11 and give it a little cc#1 (vibrato) it doesn't sound like much.
Hope you can get it singing! John