Windows 7 can play audio to only one port at a time
A limitation of Microsoft Windows 7 is that audio data can only be sent to a single output source. If both the HDMI and Audio Out ports have devices connected to them, then Windows generally defaults to the HDMI port. As a result, sound cannot play through speakers connected to the Audio Out port while a device is also connected to the HDMI port.
Note: make sure your HDMI devices are on before you start your computer.