how to creat a secret key for audio file using AES algorithm ?

i am a begginer at matlab , i am working my project in which i have to embed audio file with in a grey scale image. first of all i have to encrypt that audio file to ensure its security using AES(Advance Encryption Standard Algorithm). now i have to creat a secret key for audio file .please help me how to creat it or what are the steps to encrypt and decrypt a simple audio file using AES algorithm