You may select and upload a file to be shared with participants.
Select Media Sharing in the navigation bar on the left and click Add File to upload the file.
During Media Sharing, the user that appears in the small screen at the bottom is the one whose voice is heard (one who turns on his/her microphone and makes a sound).
Move the screen with the arrow at the bottom
(this is not applied to all, so users must move them individually!)