All stun does is help the softphone see what's the ip address and how the router NATting effects ports. So your soft phone on the laptop with out stun thinks it's ipaddress is your internal address ie (192.168.1.x) but with stun it see it's your actual public ip address. This is simplified but you get the point.

The audio quality is unaffected by stun. what is the audio quality problem's you are having?