Bloxwich teenager Beth Ford was filmed - unknown to her - singing to her mother Lesley in an empty chapel.
The footage was put on YouTube but passed almost unnoticed until it was picked up by the San Francisco Globe which described the 14-year-old's voice as 'utterly sublime' and encouraged readers to listen to the video.
The number of views has now leapt from just eight last Thursday to more than 34,000 and the film has been liked on the Globe's FaceBook page 297,000 times.
Beth, who attends Walsall Academy, said: "I'm still in shock, it's hard to believe."
The Year 9 pupil, who sang with top tenor Alfie Boe at a packed Symphony Hall in Birmingham four years ago, was filmed singing Pie Jesu, from Andrew Lloyd Webber's Requiem, at a chapel at Rugby School at Easter.
Mrs Ford, 50, a probation service officer, said: "It was such a lovely place and there was no one around, I turned to her and said I'd love to hear you sing.
"She started singing Pie Jesu. The acoustics were wonderful and a family friend was recording her on an iPhone. That's what makes it so lovely and natural, she had no idea."
The young singer, of Stafford Road, has recently launched her own website and will release her debut single, If I Could Escape, later this year. Her voice has been likened to that of legendary female vocalists Stevie Nicks, Eva Cassidy and Sarah McLachlan.
Her recording of I'm Always Chasing Rainbows, which is on her website, has been reviewed , Michael Boe, co-founder of North Star Opera and Alfie Boe's older brother, said: "So refreshing to hear a young artist really use her natural and unique voice without resorting to vocal gimmicks. This is an artist who deserves to be heard and is certainly one to watch out for in the future. Every note, spot on."
She came to sing with Alfie Boe, whom she describes as her 'main inspiration', after her parents treated her to VIP package, including meet-and-greet tickets, for his Bring Him Home tour at the Symphony Hall in 2011.
Her father Nigel, a 47-year-old solicitor, chanced his luck and asked the star whether his daughter could sing with him. To the family's delight he said yes and he invited Beth and another young singer, Flo Bannigan, to sing Impossible Dream with him at that afternoon's soundcheck.
A video of the performance shows Boe and conductor James Morgan exchanging glances of appreciation at Beth's voice. The singer was so impressed that he invited the girls to sing alongside him at the concert that evening.
Beth said: "When my friends start talking about bands and pop singers, I go 'I don't know what you're talking about.'
"They're thrilled by all the attention I've been getting over the last couple of days - I still really don't know what to think. It is exciting though. I'm just glad people like listening to the video."
She found out she had gone global after going to bed on Saturday night. "My dad woke me up in the middle of the night and said 'Beth, you've had 28,000 hits on YouTube.' I was too sleepy at the time to react."
She would like to turn professional when she is older but says:"It's not my intention to become a famous singer, it would just be an added bonus."
Mr Ford said: "The last 24 hours have been completely unexpected. It's unbelievable how many people have now watched the video. Maybe 'going viral' is a bit over the top but that's what people are saying.
"She's not into pop music and never has been. She's always liked classical and musical theatre. Singing with Alfie Boe was a dream come true for her. It was a heck of a thing to sing in front of a few thousand people like that.
"Friends have said she should go on Britain's Got Talent but she doesn't want to. She loves singing but it's not the be all and end all."
Beth, who is part of Walsall Council's Gifted and Talented scheme,
sings with Walsall Youth Choir at the the Forest Arts Centre and Birmingham's acclaimed Ex Cathedra youth choir.
Mr Ford said: "She could sing before she could talk. People have described her voice as haunting and very pure. As her father, with an obviously biased view, I would have to agree."