Can the iwatch play sound when it is out of battery?

No.

If you just want to use Apple Watch to check the time and measure your heartbeat, you can use it. But if you want to use all the functions of Apple Watch, you may still need to buy an Apple phone. Introduction to Apple Watch performance:

1. Watch has its own exclusive App. 2. The SDK releases WatchKit and opens various watch interfaces for developers to call (such as BMWi, the watch can check the status of BMW i series cars and perform simple operations, such as locking the door). 3. Apple Watch has NFC4. Waterproof 5. Supports iPhone 5 and above models, supports iPhone 5 5C 5 S and iPhone 6 and iPhone 6 Plus 3. Detects many of your motion data. 4. The two watches can directly communicate with each other when they are close to each other. 5. Detect pulse. 6. Wireless charging. Apple Watch inductive charging! Apple Watch can be used to make calls. Apple Watch can use Siri and connect to the iPhone to send and receive text messages. The watch has a built-in "digital touch". You can directly click on your friend's avatar to communicate through the most straightforward and simple drawing. All paintings will be displayed in real time on the other party's watch.