How can I use my voice in my friends twitch stream?

In the Twitch settings while streaming you can choose to mute your mic so that your voice won't be heard on your stream. Your friends will still be able to chat with you.

Regarding your second question, even with the microphone on, the twitch stream does not pick up the voices of other people in your party. I actually found this forum trying to find a way to make twitch pick up the voices of my friends as well as my own.

thank you for your quick reply.

unfortunately i cant try this until tomorrow but if it does work i will mark it as helpful

thats the problem, i dont have the option to include my audio

Well if he's streaming on twitch you need to be in a party with your friend. Once you are... open up the party where it shows who's in the party and etc.. there should be an option where it says "Include my audio" Click it and your audio should be joined in with the stream.

If you are unable to hear yourself or others on your broadcast. Please review the Privacy and Online Safety settings for your account. Check your settings and make sure you allow audio to be shared outside of Xbox Live.