New user having audio problems

Hi all

I have recently set up my WD media player. I have played a few 720p films through it, no probelm. However, I have tried 2 more films today and I get a perfect picture but no audio. I have tried switching the sound output to stero from digital but it has no effect. Am sure there is an easy solution to this, just I am new and have no idea how to implement it. 

Thanks in advance. 

What format is the audio?