Android: MegaSound is a free utility that lets you set custom notification sounds for each app on your phone. You can have one alert for an incoming SMS, another for a new email, and another for Hangouts or WhatsApp. That way you can tell the app that wants your attention before you even look at your phone.
Once installed, you can add any apps you want MegaSound to control to its list, and then tap each one to select a custom song, alert sound, or other notification sound. MegaSound only works with apps that you add to its list, and it has no effect on other apps and sounds on your phone. It's remarkably easy to set up, and it gets the job done.
The only downside is that the app requires Android 4.3 to run, so it won't work on older Android phones. MegaSound is free at Google Play. If you're looking for ways to customise your system notifications, take a look at previously mentioned Ultimate Sound Control.