As brandall wrote in this answer, there is no native way to convert audio files to text in Android. However, you could use the libraries he suggests in his answer (Nuance and A&T look good to me).
As an alternative, I would use an API to get the lyrics for the song such as chartlyrics.com.
Now you have the lyrics, but syncing them with the song is very difficult. I don't know how to do it and would display the whole lyrics to let the user scroll through.
