No audio on videos

I have a Lenovo v570 when i started to watch videos the sound goes in and out i don't hear anything then it freezes when i pause and unpause it the sound comes back but i have to keep pausing the video for the sound to come back this just started yesterday it wasn't doing this earlier how can i fix this? i was getting a message last night saying your computer is low on memory but i have 6gb

I have a video of it i tried to record it the best i could No sound on video - YouTube

I update flash last night before this happened

yes i sometimes have skype open but its closed today

I'm using firefox that's the browser i always use i just tried watching on internet explorer sound on video seems to be fine

i did the step in your post i still have the sound issue