At 15, Wolfgang Amadeus Mozart was commissioned to provide music for a royal wedding in Milan. He offered the dramatic vocal work ''Ascanio in Alba" and musical accompaniment for a ballet to be danced between its two acts. The 1771 premiere was well-received, a repeat performance was demanded two days later, and then, incredibly, the score of the intermission music simply disappeared, the only trace of it a copy of a version for piano made sometime in the 1930s.
This matter was considered closed until the Australian musicologist Ron Hunter studied that sketch, thought about the sort of orchestra available in Milan, and reconstituted the ballet music for performance by the Handel and Haydn Society, bringing about a new Mozart piece just in time for the composer's 250th birthday celebrations.
This is music to accompany a visual display, and it is lovely, but too simple to stand alone. The thrill of a modern world premiere more than made up for the lack of distraction, though. The mood at Symphony Hall on Friday night was appropriately awed. While a full house listened, hushed and expectant, onstage there was more exuberance than reverence. The first snippet of the ballet music was a charming exercise in simple counterpoint, and the third featured some brief, sweet melodies. The sixth, a joyful lilt, was the gem.
It must have been a challenge to devise an all-Mozart program to contain this, and the choice by Handel and Haydn conductor Grant Llewellyn of a choral first half (the incidental music to ''Thamos, King of Egypt") and a pastiche second half (the ballet music, plus some arias) was inspired.
The evening's only letdown came with the two unexceptional soloists. Soprano Karina Gauvin has a bland if pretty voice, and her ''Voi avete un cor fedele" was overly mannered; tenor Richard Clement showed only moderate enthusiasm in his two arias from ''Idomeneo." Much more engaging were the brief solos by chorus members who dashed out, sang passionately for a few moments, then ducked back to rejoin their sections; chorus tenor Thomas Gregg was particularly spirited in ''O voto tremendo!" from ''Idomeneo."