Thank you for the advice and replying so quickly. Since my end goal was to see how additional network overhead due to security affects voice over IP (UDP), are there any other SSL enabled applications similar to Stunnel that supports UDP?
Is this a theoretical quest, or a real-world comparison between two systems? If the latter, then compare your VOIP with no encryption, vs with encryption. How you turn on encryption? Maybe your software has a big green button, I have no idea.
If you can't actually tunnel your VOIP over stunnel, then there's no point in trying to do any performance tests using it! You can't possibly learn anything useful!
So either you're trying to do real world, in which case you should compare your real world secure/insecure options, or you're completely theoretical, and you aren't really stuck with UDP, are you?