I was looking for a YouTube link of a recording of this song on Finest Kind's Lost in a Song album. I haven't found it, but I did find this:

https://www.youtube.com/watch?v=hvPiPgACPsQ

It's a great performance, but the recording has volume problems.
Here's my transcription from the Lost in a Song CD. It's almost the same as what Burke posted above, with a few refinements. Whether he's right or me, is not for me to say.

FA, SOL, LA
(Shelley Posen)

While walking out one evening
In a little Southern town
I saw some signs upon a fence
And looked them up and down
And there among the notices
For auction sales and cars
Was a poster that read Fa-Sol-La.

Folks said that in the morning
There would be a special sing
For everyone that lived in town
And friends that they might bring
The singers each would have a chance
To lead their favorite tune
And they would sing all day up in the hall,
With potluck lunch at noon.

CHORUS:
And they would sing: FA, LA, SOL
And make the heavens ring
Like they did so long ago
Their hearts would fill with joy and love
To sing in praise of God above
And sing out with a LA, SOL, FA.

Next morning I went to the hall,
And found a vacant chair
The singers were assembled
Sitting in a hollow square
A book they called the Sacred Harp
Was put into my hand
The notes were written all in shapes
I could not understand.

CHORUS:
But they were FA — FA, LA, SOL
They helped the singers read the music
Like so long ago
Triangle, circle, diamond, square,
Those were the notes that they read there
And named them with a LA, SOL, FA.

An old man rose and walked into
The center of the square
He called for silence, bowed his head,
And said a little prayer
He thumbed the pages in his book
And called a number out
And he gave the parts their LA, SOL, FA.

CHORUS:
And they sang FA, Sol, La — FA, LA, SOL
Their voices joined in harmony,
Their faces all aglow
Each singer was a child of grace,
Soprano, alto, tenor, bass
As they sang out their LA, SOL, FA.

The singing was so sweet in heart,
It took my breath away
I sang along as best I could
Through all that glorious day
From "Wondrous Love" to "Parting Hand"
We sang them all day long
But I do believe that "Showers of Blessing"
Was my favorite song

Sol - Fa - La
He sends His show'rs of blessings down
To cheer the plains below;
He makes the grass the mountains crown;
And corn in valleys grow.

My friends if you should ever find
This world is not your home
You've lost your faith in God or Man,
And think you're all alone
Or else you've met such happiness
That music fills your heart
Whether you are feeling joy or woe,
Sing from the Sacred Harp.

CHORUS
And you just sing FA — FA, LA, SOL
And join the gospel choir
That began so long ago
In city, town or village square,
You just might find that anywhere
They're singing with a LA, SOL, FA
In city, town or village square,
You just might find that anywhere
They're singing with a LA, SOL, FA

Showers of Blessings (Sacred Harp p.528) words by Isaac Watts, 1719
Music by A. A. Blocker and Joseph Stone, 1959

Directory of Sacred Harp Singings to experience what the song is about.

Some explanations of the song:
Major scale in FA SOL LA:
FA, SOL, LA, FA, SOL, LA, MI, FA

FA, LA, SOL should be sung as major triad 1-3-5
LA, SOL, FA descending 3-2-1 of major scale.

triangle=FA
circle=SOL
square=LA
diamond=mi