Which program can I use to change Audio Files?

MP4? Do you mean convert AAC audio file to Mp4 audio file(.m4a)? if yes, you can try RZ Audio Converter, it can convert any audio files between different formats, such as convert mp3 to wav, convert aac to mp4 audio, etc, you can yahoo or google search and download RZ Audio Converter, easy to use, hope it can help you!