I thought I was tech savvy but this monitor has almost diminished me to a quivering wreck!

I sent this back 3 weeks ago as the audio failed to work, the replacement monitor's audio also does not work.....so unless the retailer has sent me the original monitor the problem must surely lie with me?

I have connected the following devices to the monitor via the supplied HDMI cable and my own HDMI cables;
Apple Mac Mini 2012 version,
Lenovo Laptop (Intel graphics card, Windows 7),
WD Media Player.

I have selected the Monitor within the Audio setups of each of the devices. But no sound is coming from the Monitor; I have checked the monitor to ensure it is not on mute. When I play a song on the laptop with the monitor defaulted as the audio device, the mixer for the device and the audio player is reacting to the song but I can hear no sound until I switch the audio back to the internal speakers. I have updated the drivers but still no joy! I have tried contacting the Philips computer helpline but they are not open at the weekend. I have googled the monitor model number but there is not too much forum information about this product. There is a HDMI audio out socket on the back of the monitor but as no cable was supplied with the product I'm presuming that this is not the reason why I can't get the monitor audio to work?!

Any help appreciated. Thank you.