Currently in Hookpad 1, I you want to play a tab in piano mode, you have to have some way to play MIDIs and optionally a soundfont on your device.
This is why, when you access a tab with a Flash-capable browser on iOS (Such as Puffin Web Browser), no sound plays when piano mode plays.
If you don’t rely on MIDI and soundfonts, in HTML5-based Hookpad 2, you could have near universal access from any device.