Hi DjRidgley,,
I already received a response from our internal support and I'd like to let you know that the automatic ear detection cannot be disabled manually because it is by design to turn off the audio when you remove or place the Surface headphone around your neck. This is for users to save battery energy. If you'd like this option to be raised to our software engineers, it would be best to send this as a suggestion through the Feedback Hub app.
If this answers your question, feel free to mark this post as an answer. Otherwise, don't hesitate to respond to this thread if there's anything else that needs to be addressed.
Kind Regards,
Crest_Z