Q: I have an iPhone, an iPod touch and an iPad. I have bought music on all three. iTunes will not let me listen to music on a device unless I bought it on that device. I thought the newest version of iTunes (10.4) would allow that. Anybody figured this out?
A: Normally, music purchased on one iOS device is transferred back to iTunes where it can be synced to other devices. This is not a new feature of iTunes 10.4 but has actually been the case since the iPhone and iPod touch began supporting on-device purchasing back in 2008. In order for this to work, however, your computer must be authorized for the same iTunes Store account used to purchase the content. Unless you’ve never purchased anything from the iTunes Store directly in iTunes, your computer should already be authorized, but you can manually authorize it by selecting Authorize Computer from the Store menu in iTunes and entering your iTunes Store Apple ID and password.
Note that for purchased content to automatically transfer to your other devices you must also be using automatic synchronization with iTunes and the new content must appear in a playlist selected for synchronization to those devices. If you are not using automatic sync, then the new tracks will appear in your iTunes library but will need to be manually copied to your other iOS devices.
A much more recent feature, “iTunes in the Cloud” allows music purchased on one device to be automatically downloaded to other devices, however this feature requires iOS 4.3 or later on all of your devices and is only available to U.S. iTunes Store users. This feature is not enabled by default and you will need to turn it on under the “Store” section in your iOS Settings app:
A similar option can be found on the “Store” panel in your Preferences in iTunes 10.3 or later, allowing apps, books and music purchased on any of your iOS devices to also be automatically downloaded to iTunes without requiring you to specifically sync your device.