There are a few ways to do this. Here's one way:
1. On your iPhone, go to Settings > Bluetooth and turn on Bluetooth.
2. Make sure your iPhone is connected to the same Wi-Fi network as your computer.
3. On your computer, open iTunes and make sure it's open in the main window, not in the sidebar.
4. Click on the AirPlay button in the upper-right corner of the iTunes window (it looks like a little triangle).
5. From the list of devices that appears, select your iPhone.
6. In the lower-left corner of the iTunes window, change the Output setting from "iTunes" to "iPhone."
7. Now, any music you play in iTunes will stream wirelessly to your iPhone! Enjoy! :)
8. If you want to stop streaming music to your iPhone, just go back to the AirPlay menu and select "iTunes" as your Output device.
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