Voice isolation is notoriously difficult. It’s like trying to extract a banana from a banana milk shake.
This sample has been produce by using the “Vocal Removal” effect in Audacity 1.3.13, then using the result as a noise profile in the Noise Removal effect, and applied to the original audio track. The Equalization effect has then been applied to remove low bass and high treble. Finally the track has been “Envelope” shaped to remove the unwanted parts. With more time and effort tweaking the effects you may achieve slightly better results (this was done quite quickly), but I’ll be surprised if you get much better.