I bought a Thinkpad Ultra Dock recently, and last week found that whenever I dock either of my x270s, the laptop's microphone stops working.
Turns out, either someone at Lenovo decided that the 3.5mm audio output on the dock should be a headphone/microphone combo jack.
So, if you have speakers plugged into the jack on the dock, the laptop assumes that there is also a microphone plugged in there and just mutes the internal microphone. You can't un-mute the internal mic in software.
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