Symptoms
When you connect a monitor that supports audio to the MiniDP port of a Surface Book or Surface Pro 4 device, you don't get any audio signal through the monitor.
Cause
This issue occurs if the sound driver is not enumerating the audio endpoint.
Workaround
To work around this issue, try the following methods:
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Restart Windows.
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Disconnect the monitor, and then reconnect it to the MiniDP port.
More Information
This issue may occur if you connect the monitor while Windows is running, or if you connect it while the device is sleeping or hibernating.