TwitchTv in game sound does not stream

I am experiencing the same issue

Must be occurring only on Mac, as I have no problems on Windows 8.

This is not a bug, it is just a limitation due to the way mac configures its audio setup. To fix this issue you need to get a program called soundflower. You can find a guide here : https://help.mojang.com/customer/portal/articles/1374877-configuring-soundflower-for-streaming-on-apple-computers