If you are trying to convert speech from an audio file (such as the MP3) to a written text, you need to use a special transcription service (speech to text converters).
One of such great services is for example VoiceBase, using which users can upload their audio files and get transcripts as text files. You can register account for free an get 200 audio and 20 video hours of computer transcription for free, with an option to order human transcript.