It's fair to say making music on mobile apps has never been easier than it is in 2013. App sales are projected to hit $16 billion this year, per Wireless Expertise, and the market is only expanding. For musicians and budding producers, that means a treasure trove of apps to get into and new sounds to make. Here are 20 apps for Android and iPhone to get those creative juices flowing. Who knows, becoming the next Pharrell Williams or Skrillex could soon be a reality.