Get a portable sound dock for your wife that she can lug outside with her MP3 player, CD's. Very easy for her to setup, won't need to fiddle your main system and it will provide her music at her finger tips. A sound dock would be able to provide sufficient loudness levels (avoid Bose though, which suffers from noticeable compression at higher levels) at her parties since she can setup directly were she is entertaining and can move it around if the guests go to a different area or to a room inside.
It will be her own little toy and I bet she will love it.
