Creative and Cultural Skills and Equity, two industry bodies, say:

Sound designers work closely with directors and with the stage management team, and in some cases with performers, or musical directors and musicians, to design & create the production’s sound. They may suggest appropriate sound effects, music, and performance sound levels, in order to enhance the production.

They research, acquire and compile appropriate effects and music, transferring these as necessary onto appropriate (generally digital) formats for playback (usually referred to as sound ‘tapes’). They prepare music and effects sound plans, and equipment plans, which may be for their own use or for others. This documentation outlines all sound requirements, including sound effects and music, equipment specifications re different types of microphones, loudspeakers etc.