Please help: Macbook pro (2012) -TV via HDMI. Sound problems

Hi! I have a problem with the sound when I connect my Macbook pro (bought in 2012) to a Phillips HD TV. The strange thing is that it only happens with some videos (specially youtube videos) but the sound works perfectly with films and audio files I have in my computer, and also when I watch TV online (Eurosport player for instance and other similar applications). It's even stranger because it doesn't happen all the time; last week I tried many times to watch some youtube videos and after unplugging the HDMI cable it seemed to work. As I said, with Eurosport player and many films downloaded in my macbook I don't have this problem (sound is ok all the time), and this is why I'm really confused. I suppose it's not a connection problem, otherwise I wouldn't have sound in any video, right? I've tried two different HDMI's cables but nothing has changed. Please, someone help me, as I can't figure out what's happening. Thank you very much!