(a very good musicology question suggested by BenV, from the definition phase of the "Classical Music" area51 proposal).
Beethoven's ninth Symphony (opus 125) is one of the most well-known works of classical music. His last movement uses a choir together with the orchestra.
It is usually considered the first "choral" symphony. Are there no other examples or forerunners of this idea before 1824?
Beethoven's own "Choral Fantasy" came before the Ninth Symphony. It's a weird (and wonderful) piece composed for a particular occasion (actually composed to fill out a concert that was already far too long).
The piano starts it as a solo and plays an improvisatory cadenza that's three or four minutes long. Then the orchestra quietly starts in, builds up in volume, and plays a tune that sounds very much like the Ninth Symphony theme, followed by variations. Finally the chorus enters for a big finish.
That was in 1808. In 1820 composer / pianist Daniel Steibelt wrote and performed a piano concerto with a choral finale, still a few years before Beethoven's Ninth.
In short, a lot of strange things were going on around the turn of the century. The word "symphony" itself changed its meaning from a generic piece for many instruments (like the "Pastoral Symphony" in Handel's "Messiah") to a specific form. In Handel's day, calling a piece a "choral symphony" would be like calling it a "piece for instruments and no singers with singers". For centuries there have been pieces for chorus accompanied by orchestra, but we may never know who was the first to write a piece for orchestra accompanied by chorus.
I have to say no.
As you said, Beethoven's 9th symphony was the first choral symphony but it was not the first piece to combine chorus and orchestra.
There were already pieces from the Baroque that combined them, as the same definition of cantata says:
Wikipedia, following the New Grove Dictionary of Music and Musicians, reports correctly that there was a choral symphony published by a now obscure composer, Peter von Winter, in 1814, ten years before Beethoven's 9th. It is called the Schlachtsymphonie or "battle symphony."
Beethoven also introduced a chorus at the very end of his Fantasy for Piano, Chorus, and Orchestra, written in 1808.
Let's make a distinction here. A symphony is a composition in a specific type of musical form written for an orchestra, which is a type of musical ensemble.
To state it simply, a symphony is a very specific type of long song, whereas an orchestra is a type of band.
There are many kinds of musical pieces written to be played by orchestras that are not symphonies. These include concertos, sonatas, suites, oratorios, and cantatas.
These days the term symphony is used incorrectly to describe an orchestra. People say "I'm going to see the Atlanta Symphony" when what they mean is "I'm going to see the Atlanta Symphony Orchestra". The Atlanta Symphony Orchestra is an orchestra that plays symphonies, as opposed to an orchestra that plays ballets or an orchestra that plays operas or an orchestra that plays oratorios or cantatas.
Calling an orchestra a "symphony" is a modern mistake, something that's appeared in the last thirty years or so.
Here's my point: Yes, Beethoven's Ninth is, basically, the first symphony to feature a chorus.
There are many pieces going back centuries before Beethoven that feature an orchestra and a chorus, but those musical works are not symphonies. They are oratorios, or operas, or ballets, or other forms. Bach's B-minor Mass features a chorus and an orchestra, but Bach's B-minor Mass is not a symphony, it is a Mass. Handel's Messiah features a chorus and an orchestra, but Handel's Messiah is not a symphony, it is an oratorio. Beethoven's Choral Fantasy is not a symphony.
And by way of establishing my credentials, I'm the volunteer business administrator for the Atlanta Baroque Orchestra, which has been performing for fifteen years. They are an orchestra, but they only rarely play symphonies. They more often play concertos, suites, cantatas, and other forms.
Ignoring the aspect of the question that deals specifically with symphonic forms (which Mark Lutton has already answered), the answer to your question depends somewhat on what you consider an "orchestra".
In the Renaissance, musical parts were written out in full polyphony, but instruments weren't usually specified. The same piece could be sung by an ensemble, played by a viola consort, or a recorder consort, or any number of other combinations of instruments. Or one line could be sung, and the others played on a lute or keyboard. Church music was often accompanied by cornetts, sackbuts, and organs. But these typically act as doubling instruments, and do not have independent parts.
When we get to the Baroque, and look for genres that include voices and instruments, we immediately come to opera, the invention of which has been claimed to mark the beginning of the Baroque.
The first opera, Dafne, was written by Jacopo Peri, and first performed in 1598. Although it is now lost to us, it was apparently scored for harpsichord, lute, viol, archlute, and recorders.
The first surviving opera, Euridice, also by Peri, was first performed in 1600. According to the book Euridice: An opera in one act, five scenes, Peri lists four instrumentalists (playing harpsichord, lira da gamba, bass lute, and chitarrone [theorbo]) in the Preface, but does not imply they were the only ones. The book suggests that other instruments must have been used -- probably recorders, and perhaps violins -- but admits "we shall probably never know the exact composition of this first opera orchestra, nor precisely how the instruments were used."
Another opera named La Dafne was written by Marco da Gagliano and first performed privately in 1608. I don't know what the orchestral forces for this opera were, but wikipedia mentions that, in his introduction, "Gagliano recommends clearly separating the soloists from the chorus, [and] positioning the orchestra in front of the stage so the singers can see them properly," indicating that there was some sort of combination of chorus and orchestra.
These are all mentioned as precursors to what is probably the best answer to your question -- Claudio Monteverdi's opera L'Orfeo (youtube with score), which premiered in 1607, was published in 1609, and is the earliest opera that is still frequently performed today. In it, Monteverdi lists out very specifically some 40 instruments that he wants used in it's performance (though not all at once, so the number of instrumentalists can be smaller). These include various groups of strings, brass, winds, and continuo instruments.
The staves don't generally name specifically which instruments play which staff, but many of the movements have headings that indicate which instruments are to accompany the voices. For example: "This ballet is sung to the tune of five violas, three theorbos, two harpsichords, a double-harp, a bass viol, and a sopranino recorder." Or, "Charon sings to the sound of the regal" (a regal was a type of small reed organ). There are also specific instrumental obbligati, such as calling for a pair of violins or a pair of cornetts to play specific figures that are independent from the voices. In addition, there are several instrumental-only movements interspersed throughout (including, ironically, some that are labelled "Sinfonia", in accordance with the early usage of that term that Mark mentions).
Given the size and diversity of the forces involved, and the relative specificity with which they are employed (compared to other works that preceded it), I'd have to say that L'Orfeo is probably about the closest you'll get to a "first" work for an actual orchestra, as opposed to an ensemble. And since it also includes a choir, this piece is probably the answer to your question, or as close as you're likely to get (certainly within a decade).
Everybody now knows about Beethoven admiration of Handel and his music and the influence of Handel in the last period of Beethoven music. In fact, musical critics in Beethoven times critisized the "Baroques extravagant ideas " of the last quartets, the Missa Solemnis and the 9th symphony. I am not saying that Handel wrote the first symphony with a choir, but surely he influenced Beethoven's idea of Drama in music. Sure, Saul or Theodora are not symphonies but they stand out as superb examples on how to convey the idea of drama and musical emotion mixing music and voice.
