"I sometimes think that if I had a beard and I was old and fat then people might take me a bit more seriously," the English "wunderkind" told AFP as she played on the piano excerpts of the work.
But this attitude, she adds, has changed since preparations began in this hallowed former stamping ground of Mozart and Schubert for the December 29 premiere of "Cinderella".
She started writing the two-hour opera when she was eight, and as well as composing the music she has given the traditional folk tale her own twist, setting it in an opera house.
Thus the eponymous heroine writes a tune that perfectly fits the handsome prince's poem, while the evil stepsisters are "pompous prima donnas," the young composer explains.
It takes place in an imaginary country she calls Transylvanian, populated by imaginary composers.
Her favourite, Antonin Yellowsink, wrote one of the more melancholy melodies -- "and I stole it", she joked.
But according to Anna Voshege, an Australian singer who plays one of the stepsisters, what Alma composes is anything but childish.
"I heard the music before I knew she was 11, I was really quite shocked. Some of the nuances in that music are really very special, very intricate," Voshege told AFP.
"It's very beautiful music and she is far beyond her years," she said.
'Something special' at four
Alma's father Guy Deutscher first realised his daughter was out of the ordinary when she asked, aged two or three, "'How can music be so beautiful?'," he recalled.
"When she was about four, when she started inventing her own melodies, I think that's when we noticed there was really something special," he told AFP in Vienna where the family from southern England has been spending time ahead of the premiere.
At six she composed her first full piano sonata, at seven a mini-opera called "The Sweeper of Dreams", and at nine a concerto for violin and orchestra.
A chamber music rendition of "Cinderella" was performed in Israel last year, but the Vienna version, with Indian conductor maestro Zubin Mehta as patron, is much longer and will be a proper opera.
She has won praise from renowned conductors like Daniel Barenboim and a "bowled over" Simon Rattle. Last year she appeared at a Google Zeitgeist conference with the likes of Stephen Hawking.
But, her dad says, Alma also has a very normal life. "She loves climbing trees and running around like every other child," he said.
Modern music though is "too loud" and not her thing, she says -- not even Justin Bieber or The Beatles. "Oh no, I don't know any of them!"
Special skipping rope
Alma gets her ideas when she's about to go to sleep or when she's waking up, when she's messing around on the piano -- or by her own special method, as she explains.
"Another way of getting my melodies is by skipping with this skipping rope," she says, proudly and earnestly showing off her prized possession.
"I don't actually skip but I wave it round like this and I tell stories in my mind. Very often a melody just springs into my head. And then I run back and write it down in my notebook," she says, bursting with enthusiasm.
"You see it has to be just this kind of skipping rope, with shining tassels and sparkles. Other ropes don't work at all."
But she plays down the inevitable comparisons with Mozart, who was also just 11 when his first opera, or sacred drama to be precise, was first performed back in 1767.
"I prefer not to be compared to anyone, to write my own music, just to be a little Alma... Because if I just wrote everything that Mozart had already written it would be rather boring!"