In order to capture your voice audio on iOS, please make sure both your microphone audio and in-game voice chat are on before you start streaming.
If the audio stops or the volume becomes very low during your iOS stream, please try the following:
Step 1. 3D touch (stronger long-press) the music area in control center
Step 2. Tap ▲ on the screen
Step 3. You should see options for iPhone, Speaker, etc.
Step 4. Select Speaker
In other cases, such as when your audio cannot be heard on-stream despite turning on your mic, please try restarting the stream or rebooting your device.