Reported in https://wordpress.org/support/topic/plugin-does-not-function-on-mobile-devices?replies=1

Scroll to top functionality is working fine, however mobile devices prevent audio play as it's currently implemented in the plugin. Reproducing in Android and iOS devices.

Needs a working solution to be compatible, at least, with the majority of mobile devices on the market.

Look into solution suggested at http://pupunzi.open-lab.com/2013/03/13/making-html5-audio-actually-work-on-mobile/