Now her audience is a class of 24 six-year-olds in Brecon, Powys. But she says her classroom practice - celebrated when she won BT's teaching award for primary teacher of the year in Wales last month - has been enriched by her army experience.
"As a musician you stand up in front of lots of people and perform," she said. "As a teacher, every day is a performance with the children."
Mrs McCrohon, 33, who has taught at Mount Street infant school for four years, was nominated for the award by Tessa Waite, whose son Alastair is in her class.
"She's dynamic and makes learning fun," said Ms Waite, an artist who has volunteered in the school and seen her teach first-hand. "She's very creative - not in an obvious, arty way, but in the way she uses different tools to communicate with the children."
Mrs McCrohon says she just aims to give young children as many different learning experiences as possible.
As maths co-ordinator, she organised a recent outdoor maths day. The children used logs to make number lines, and built a hundred-square from numbers written on pieces of a chopped-up wardrobe and hidden in the woods.
And to help children learn their sums, she uses an icing sugar pen to write sums on some biscuits and answers on others, before distributing them among the class and letting the children mill around to find their pair.
And of course music is used at every opportunity.
"We have little jingles to help with numbers, English and spelling - even songs for putting our clothes on before going out," said Mrs McCrohon. "It helps them learn without realising it."
Some ideas are more hazardous than others. This term the role-play area of her classroom is a hot air balloon, so she bought a real one on ebay and set it off in the school hall.
The balloon rose, but the meths nearly set fire to the hall floor. "The kids loved it," she laughed. "It was worth it to see their faces."
Mrs McCrohon, who grew up in Merthyr Tydfil, left university with no desire to teach, but changed her mind after participating in school concerts and workshops with the army band.
Wanting to spend more time with her policeman husband, Daniel, she left the army and took a postgraduate certificate in education at Reading university before returning to Wales.
She credits Gill Daniell, a Reading music teacher, for inspiring her classroom practice, and Raye Scott, her current teaching assistant, for keeping her enthusiasm alight.
Mount Street takes a lot of army children from the nearby Brecon army camp, and Mrs McCrohon says her former life helps her relate to them and their families.
"It's nice to be able to empathise with them, but the children can't believe I was in the army," she said. "I have to bring in photos to prove it."