Per user voice volume control in the quick menu when selecting a user does not work in the new steam audio beta.
  1. Open small menu, "select user" button, select a user
  2. Drag volume slider for that user, it does not change the users voice audio. Setting volume to 0 does not 'mute' the user and setting any other volume percentage does not change the users voice audio volume.
Muting voice via the mute button still works. I have confirmed the issue with others on the steam audio beta.