No sound after making a Facetime Call on Macbook Pro

Hello everyone,

I have a mid 2011 macbook pro with Mavericks installed. Whenever someone calls me on Facetime on my Macbook there is a strange sound coming off the speakers like they are broken. I was scared at first but I thought there is only a problem with Facetime. However, today after I made a call, speakers stopped working for good. I restarted my macbook and there were still no audio. I found a solution by making a hard restart. I am scared now to use my Facetime again on Macbook. Is there anybody who knows why this happen? This issue started when I installed Mavericks I think. Also sometimes, I cannot do any facetime calls due to the strange bubble sound out of the speakers..