I would recommend as the essence of Taoism the above book of Lao Tzu ( Hua Hu Ching), also 'Zhuangzi' and 'Lost writings of Wu Hsin'- as for latter-great work in my opinion.
As for audio, rather musical compositions than audiobooks, something like:

I guess it may be composed by one of the 'Seven Sages of the Bamboo Grove' but not sure, you'll have to research into it.
:)