macbook air has no sound while playing powerpoint slides

I downloaded a powerpoint from my uni lecturer and it wouldnt play audios. I installed the flip4Mac and flip4Mac.wmv already and still cant hear anything when I click the play button. The strange thing is that my friend got the same issue and all he did was to restart his macbook pro and it worked, but it didnt work on mine no matter how many time I restart. Me and my friend are even using the same crack version of microsoft office..please help!!>.....