AC-3 ACM Codec Help!

Ok so here's my problem, I have just downloaded an AVI video file from a torrent. The video is fine and plays perfectly, however the audio is not there and under further inspection the audio format is an AC-3 ACM Codec. I have attempted a number of times to convert it to an MP3 format and a few other formats but to no avail.

Can anyone please assist me in using, retrieving or converting the AC-3 ACM Codec to an appropriate format with audio. Thank you.