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.
This issue occurs if the sound driver is not enumerating the audio endpoint.
To work around this issue, try the following methods:
Disconnect the monitor, and then reconnect it to the MiniDP port.
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.