problem with f5 key, problem with f5 key

I have an I Mac( 2.5 GHz Intel Core I5) with an operating system version 10.8.2. Today while watching a video on MKV player I have pressed F5 key which decreases the decibels. I have pressed the key repeatedly and it reached zero level. Since then I am not able to listen to the audio in MKV player while watching the video, though I am able to listen to the sound of key board and other sounds. I am unable to restore it to the previous level. Please suggest me how to increase the decibles by pressing F5 key or any other alternate way.