Play/Pause as an on/off toggle is still contextual within the music app. The play button is always going to be in the same place, and the pause button not much farther away. Music apps have also started displaying triangles over the album art.

Hamburger menu is an abstraction. Part of the blame here is on operating systems and their branded menu's (think Windows Start or OSX Apple menu). There's no standard to it, which is why native apps are a lot easier to navigate because they conform to what the user expects. That fragmentation grows when you divide by the user's browser, device, etc. It's just not standard and not contextual enough to be a standalone icon.