I note that both the speakers and the headphone are active at the same time. Suggestion is to mute the 6400M internal speaker when I plug the headphones in the rear-panel headphone jack per 29.3 of the User Guide.
I would think it is best to be user selected. Sometimes while operating I remove my headset for just a moment and I can still hear what is going on in the conversation. If it were an automatic audio cutout I would need to reach behind the radio to unplug the headset in order to listen to the speaker. I am sure it is not to bothersome to simply click the speaker mute.
As Bill suggests, just press mute next to the Master Volume, that will mute speakers, then all you have (if it isnt muted) is the headphones. With the connectors on the back of the radio on the new models, constantly reaching to the back to plug or unplug is not an option.