When you mute someone on a channel, there are several ways to unmute him:
1. Go to channel message history, locate any message from that user to a channel, tap and hold to call context menu. In a menu click "Unmute user"
2. When muted user talks into a channel, a message notification is shown at the bottom of Zello app window. If you are already on a channel talk screen, clicking on that notification will unmute muted user. Note, if channel talk screen is not active, or a talk screen for a different contact is shown, clicking on notification will open talking channel screen for you but won't unmute the user; you will need to click second time on the notification to unmute talking user
3. Channel user list also allows to mute/unmute users using context menu (by pressing and holding user item), but this feature only works if you are not a channel owner or moderator.