Another new feature allows the speaker phone to turn on automatically once you move the handset away from your ear while listening to a phone call. The Audio Routing option should end up on handsets belonging to Windows Insiders sometime over the next several weeks.
The Windows 10 Mobile update started getting pushed out to Windows Phone handsets last week. As we recently told you, 50% of the active Windows Phone handsets are going to be sitting out the update. That includes the most popular Windows Phone of all time, the entry level Nokia Lumia 520.
Audio Routing is a feature that will come to the internal Redstone builds of Windows 10
source: mswin.me via MSPoweruser