Assigning Sounds

There are multiple ways to select a Sound for an Instrument.

Rack Module Library
Search and browse the Rack Module Library on the left sidebar that contains rack module presets, plug-ins, MIDI ports, Drones and global instruments.
Drop any available item on a Track to create a Rack Module and select one of its sounds.
Drop items below the bottom track to append a new track.
Anywhere you see this icon, you can open the Sound Wizard. Alternatively you can open it from the menu with Instrument > Assign Sound .... It guides you through the process of setting up or extending a Rack Module and creates a Device Description where one is needed, so you don't have to. This method is best when you need to add new sounds to an arrangement.
This icon shows up on tabs related to sound assignment, for example the Parameter Inspector on the right sidebar. There you can select an existing Rack Module and channel from pop-up menus to use a sound provided by that module. This method is convenient for selecting fixed-channel sounds you have already added to an arrangement.
Copy From Other Arrangements
Drop any Rack Module from another arrangement's rack on your Arrangement Rack. Sounds are then available for the Sound Wizard to browse.
Manual Setup
Visit the Arrangement Rack and add a new Rack Module and set it up as explained here. Sounds are then available for the Sound Wizard to browse.
Important: When you select a different sound for an instrument, it adopts the sound's Playing Ranges by default. This will likely also change its current MIDI output. You can disable this by deactivating the chain link on the wizard's last page, or globally with Playback > Audio/MIDI Setup > MIDI > Sound Management. If you do so, you must verify yourself that your instrument's ranges don't violate the capabilities of the sound.

Multiple Instruments Using The Same Sound

Assigning the same sound to multiple instruments will make them play on the same MIDI channel. If you want one or more of them to play on different channels, you need to multi-select the desired instruments and do Instrument > Demand Separate Channel.

Note: This only works for devices that select sounds via MIDI program change messages on more than one dynamic channel. Sounds on fixed channels, as the name implies, are always tied to the same channel.