No Audio with media player

Hi All
I am using windows vista and trying to play an avi file. The file plays video but no sound. It doesn’t come up with any errors or anything. I have analysed the file with Gspot and it says I have 3 installed audio codecs. The type of file is AC3 (0x2000)and the three codecs I have installed are pinnacle ac3 encoder (2 channels), Pinnacle AC3 decoder, Pinnacle audio codec. of these 3, Gspot has an arrow next to Pinnacle AC3 decoder. Does anyone have any idea what is going on?