In Desktop/Web, visit User Settings (gear icon) under Notifications. Go to the section of sounds and select the Disable All Notification Sounds box. Below this option you can also tell individual sounds on or off such as message, user join or pings.
To keep Discord off your list of startup applications, you should turn off Open Discord in User Settings/Windows Settings/Mac Settings. It can also be turned off by using the start up space of Task Manager (Windows) or the start up items (Mac).
