Click on the mic icon on the top right corner. This should temporarily show the global audio mix. On the left, you have a slider for audio input, on the right one for the level of instruments in the recording. To enable voice monitoring, you need to first ensure you are wearing headphones, otherwise it will generate a shrieking feedback, as audio output gets captured back in the audio input and amplified in a loop.
To avoid this happening, we have made it so that you need to first click on the headphones icon and then only on MONITOR. If you don't do that, you get the following message: