This conversion is mostly related to attempts to transcript the speech from some audio file (for example .mp3) and save it in text format (.txt), in essence turning audio (speech) to text.
You can do this, but you will require the services of some special transcription platform, for example VoiceBase or Speech Pad. Try to search for digital transcription software, transcribe audio to text, mp3 to text converter online or similar search strings to get appropriate software or online service.
The resulting text file may require some manual editing though, as no programmed transcription service is perfect and almost always requires human input.
Updated: March 20, 2017