The Stream Deck marketplace offer sets of icons, many are free, others you purchase. Then you can add these buttons to represent what that button does.
Elgato Marketplace, some icon sets (of very many sets available)
Selecting a button and adding an icon using the SD software, shows the sets to choose the icon from.
In one of the Stream Deck threads here at this forum, is a set of Dorico notation/music related icons if I recall correctly, ideal to use for your own setup.
Edit: found it!