Paying an acoustics consultant a basic fee for a
seat-of-the-pants recommendation
might be the most effective money you can spend.
Churches with amplified music have similar needs to venues for
called dances -
complete coverage of the congregation, high intelligibility, and at
modest volume.
Most curtains have minimal sound absorption.
Thick, heavy stage curtains that are pleated will be a bit more
effective.
For an idea of what might be helpful, consider this for a 3'
wide window:
a 6' curtain rod and a fabric width of 12', with the curtain rod
well above the window and the curtain hanging well below the bottom.
With the curtains closed, you have good pleats, and with them open
there is
still some useful absorption from the 1.5' width at each side of the
window.
Most fire retardants are pretty funky. PFAS, etc.
I suggest buying fabric that "does not support flame spread".
In your backyard: https://www.guilfordofmaine.com/
They may be able to suggest an acoustics consultant.
An effective sound system for your venue, with a competent
operator, is another consideration.
Loud fans and heaters can mask intelligibility.
In 2012, The Old Farmers Ball in Asheville spent about $9000
installing 21 sound panels,
which made Bryson Gym usable.
Forty panels had been recommended...