This is very easy to do. Every music file editor should be able to do this. In fact, Music Editors must convert the MP3 file to a WAV file (or the equivalent) before they can edit it.

WAV files and MP3 files are just different types of sound files, just like TXT files and DOC files are different types of documents. Just like a word processor such as Microsoft Word can open a file of one type (e.g., TXT) and save the file as another (e.g., HTML, DOC, RTF), a music editor can do the same thing with music files.

In CoolEdit 2000, for example, you "Open" the MP3 file, then do a "Save As" and select type "PCM raw data (*.wav)" to get a WAV file.