It seems that mp3 to txt conversion is mostly related to attempts of transcription of the speech from audio file (for example .mp3) to a text file (.txt), in essence turning audio (speech) to text. You can achieve this with some special transcription platform, for example VoiceBase or Speech Pad. Try to google a bit 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 will most likely require some manual editing though, as no programed transcription service is perfect and almost always requires human input.Open MP3 file Open TXT file
A voice recognition and converter software
A Voice Search & Transcription Platform