I've always understood that STUN doesn't affect audio quality, but the audio was really choppy when I included the STUN server (zeus.voipwelcome.com) in the configuration settings. By comparison, I didn't have these audio problems when using a SIP app on my iOS device which is not configured to use STUN.

At any rate, I was able to search these forums and the forums at broadbandreports to finally figure out the correct settings without having to use STUN. In summary, I added ":5061" to the end of the domain name ("sip.voipwelcome.com:5061") and, in the Topology section, I set the Firewall Transversal Method to "Auto-detect (ICE, recommended)".

Now X-Lite works great and I don't notice the choppy audio that I was experiencing when I had it configured to use STUN.