It is actually very important having a program through which you can convert from one extension to another, since you may have to submit your song to a certain musical authority and they then may have rules for the submissions, such as that the file has to be of a certain type.
You can actually get decent material for free. See:
1) Audacity: It is probably the best one because, first of all, it is free, but it also allows you to do a number of things that may be of high interest, such as decreasing noise levels.
2) Power Sound Editor: Also free. Once more, you can do several things with it.
3) WavePad Sound Editor: Not so free. You can try for one month for free. You have to specify the directory for your files to be in. I did not find out about that until after some one hour working with it and trying to save files. Basically, it will show the files inside of it as if you have saved them, but, if you do not have a valid directory specified there, on Options, then you will never be able to find the output of your efforts.