Actually you can convert rar files into mp3's with programs like XRecode.
What, even if the .rar contains Word documents?
To try and make things quite clear:
.rar is an archive format. It is not a music format.
You cannot convert it to anything, except maybe another archive format like .zip. (Which would be fairly pointless).
You can OPEN it and extract the contents, which may be mp3 files or may be some totally other sort of file.
Opening a .rar is as simple as double-clicking it if you have 7zip installed. You can then drag the contents to wherever you like. It is quite common in our archive for the .rar to contain more than one type of file; for instance, a series of .mp3 files and a .pdf describing them.