I have this same issue with my Sony headphones.

1. Open Utilities -> Audio Midi Setup 2. In the left pane click + and add a new aggregate device 3. Add your internal mic to the aggregate device 4. Go to System Preferences -> Sound and select the aggregate device as your input

Unlike the 'raw' internal mic device this one should stay active when you disconnect / connect your headphones.

I've never found a better solution than this :( Kind of shocking that this is required - but it works.