dbpoweramp will do just about any type of conversion you need, and will do it simply and quickly. There is a link for the FREE version 10 on this page, but a google search will find any number of others. I think you'll find it to be just what you need. http://www.videohelp.com/tools/dBpowerAMP
It's what I have used for years, and found it to be quite satisfactory.