Once you tap the video, sound will turn on and the video will continue to play in an expanded view. If you turn the video to landscape mode, the sound will automatically turn on and the video will expand to fill the screen.
You can choose to revert to the previous click-to-play experience or have videos autoplay only when you’re connected to wifi via the settings.
Twitter notes that if you’re somewhere with high data rates or you have low bandwidth on your device, they'll opt you out of autoplay to avoid unexpected charges or slow performance
Check here for more details on how to change your settings.
You can download Twitter from the App Store for free.