Conveniently, each sound or image has its own node, so it can easily become part of the graph structure. An example of what the structure can look like:
Go to Design >> File Assets. If you have already uploaded some multimedia files to your Space, you will see their complete list in the left panel.
Click on "+" in the upper right corner (the red circle in the screenshot below) to create a new empty item.
Click on Browse and select your file, then fill in the Name field (what the file should be called in your Space). If you want, you can also add a description.
Click on Save. Check if the file has been added to the list in the left panel. If not, try refreshing the list by clicking on the Refresh button next to the "+" button.
Now you can insert your file into any dialogue in your Space!
When editing a dialogue, select the "Multimedia" tab (the red circle in the screenshot below).
Choose one of the files from the list and drag and drop it into the graph.
Link it to other nodes in the graph at your pleasure. The file will be played/displayed at the corresponding moment of the dialogue flow.