Google recently added a new feature to the dictionary OneBox called "audio pronunciation." Earlier, this feature was available only when you clicked "More" to read all the definitions, but now it's more accessible.
Google uses Flash to play the audio file, so the feature doesn't work if you disable Flash.
It's interesting that Google shows the audio icon if you use an iPad, even though the device doesn't let you install the Flash plugin.
The HTML5 audio tag is a better option for iOS devices and for the browsers that support it.