"This is where I'll be conducting from," says Hall. "Then we have the choir members on these four levels."
He points across the galleria where 437 singers line four balconies of the complex's eight-story parking garage.
This massive choir will open the international GALA Festival on Saturday night. The Gay and Lesbian Association of Choruses (GALA), a global consortium of LGBT choral groups, organizes the 23-year-old festival and this is the third time Denver has been the host city. GALA Choruses commissioned Hall to create a new work for the festival's opening ceremonies.
"They wanted a piece that was a tribute to Colorado," says Hall, "something that could welcome all of these other guests from around the country to Denver."
The piece is titled "Mountains and Rivers," and it could welcome up to 7,000 festival attendees.
As the title suggests, Hall says he drew inspiration from the Colorado landscape. Positioning the singers along the multi-level parking garage, he says, captures that imagery visually and sonically.
"There's kind of a shimmering effect," says Hall. "We can have people sing in a wave formation. So they start on the left-hand side and you can hear their voices move spacially across the parking garage."
Crafting music for hundreds of singers in this space has its challenges.
"So obviously, there's a bit of reverb in here," says an organizer addressing the singers on a loudspeaker. He asks the singers on each level if they can understand him. "We have to talk very slowly so it's not really reverbing and you can't understand."
Hall says the space's acoustics are "actually pretty good for choral music." He says the biggest task was taking all of the different skill levels of the singers into consideration when composing.
"We've got people who are really seasoned musicians and then people who just love singing for the joy of it but they may not read music totally well," says Hall. "So you have to think about all their strengths and weaknesses."
Hall also incorporates trumpets and handbells. Birds and the sounds of nearby traffic add some unplanned accompaniment. There's an element of spectacle to it all.
Beth Wenski sings with Denver Women's Chorus and Harmony, a Colorado chorale. She's one of the 437 singers and says the experience has had an emotional impact on her.
"It's just kind of one of those epic artistic moments," says Wenski. "You get to be with your friends from all these different choruses and deliver an opening ceremony to people around the world."
The music attracts the attention of foot traffic from the surrounding streets.
"We heard some good sound, so we thought we'd pop by," says Russ Meyer of Columbus, Ohio. "It's very creative. How cool."
Several dozen other people pause inside the galleria. Many pull out their cell phones to take photos and record videos of the multi-tiered chorus.
Hall spoke about his composition with Colorado Matters host Nathan Heffel.
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Editor's note: A previous version of this story said this was the second time Denver has been the host city for GALA Festival. This is actually the third time the event has been in Denver.