Vocal Audio Extraction

Hi, I have several audio files, .amr type, which I am looking to extract the voice audio from. some of the recordings are distorted and some with very low volume vocal. Each file is between 1 and 5 mins in length, but with audio is only small section.
I have attached an Audacity sample project which is a cutting from 1 of the audio files. I have approx 25 of these files.
thank you.