The list, compiled by a panel of 144 DJs, critics and writers, highlights 15 rising acts for the coming year.
It includes Canadian R&B sensation Alessia Cara, singer-songwriter Frances and witty rap collective Section Boyz.
Last year's victors were pop trio Years and Years, with previous winners including Adele, Sam Smith and Haim.
After topping the list, Years and Years' debut album, Communion, entered the charts at number one and the band recently announced their first headline show at Wembley Arena.
2015's runner-up, singer-songwriter James Bay, also scored the year's biggest-selling debut album, Chaos and the Calm.
"The fact that people think I can have the same potential as these huge artists is incredible," said Cara, whose debut single, Here, has just entered the US top 10.
Find out more and watch videos on the Sound Of 2016 site
What happened to the previous winners?
As with previous years, young solo artists feature prominently on the 2016 list, including soul diva Izzy Bizu and British-Kosovan pop singer Dua Lipa, who was talent-spotted by Lana Del Rey's managers.
But the front-runner is bound to be Jack Garratt  - a soulful multi-instrumentalist who has been compared to Ed Sheeran and James Blake. The 24-year-old, from Buckinghamshire, has already won the Brits Critics' Choice award, and will receive the Introducing prize at next month's BBC Music Awards.
Guitars are notably absent from the longlist, with Stockport psych-rock quintet Blossoms and indie upstart Ratboy the only traditional rock acts to make the cut.
"We are delighted to be a part of the BBC's Sound Of 2016," said Blossoms. "To discover we're one of the only guitar acts on the list this year is exciting for all of us. Viva Stockport!"
As with last year, rap and grime make a strong showing, with Londoners J Hus, Loyle Carner, WSTRN and Section Boyz all featured.
Most of them reached the longlist without traditional music industry support. J Hus has racked up six million streams of his Afrobeat-inspired party track Lean and Bop without a record deal; while Section Boyz saw their independently-released mixtape, Don't Panic, enter the top 40 earlier this year.
"I am so excited to hear that I am in the longlist," said J Hus. "It's a massive honour to be recognised and makes me want to continue for the fans."
As the daughter of pop legend Neneh Cherry and Massive Attack producer Cameron McVey, Stockholm-born Mabel was raised around music - but held back from her own career until recently.
"It's so unoriginal of me. Why didn't I just go and be an astronaut?" she joked in an interview with The Beat Juice.
Similarly, London-born NAO was reluctant to share her music, despite a degree in jazz from the Guildhall School of Music & Drama. She worked as a backing singer for the likes of Jarvis Cocker and Pulp before gathering the courage to upload some of her music to SoundCloud. Shortly afterwards, she found her first single, So Good, being championed by Zane Lowe.
The list is rounded off by electronic producer Muru Masa and 16-year-old folk singer Billie Marten.
The Sound Of... list launched in 2003, and has a track record in championing new and innovative acts, many of whom have gone on to achieve global success.
According to the rules, the acts should not already be well known to the UK general public - through featuring in the final stages of a TV talent show, for example, or having already been a member of a successful band.
They must also not have been the lead artist on a UK top 20 single or album before 30 October 2015 - though guest vocalists are eligible.
The 15-act longlist will be whittled down to five in January, with the winner announced on the BBC Radio 1 Breakfast Show on 8 January 2016.
DJ Annie Mac said the Sound Of 2016 was "a definitive list of the key artists to look out for and listen to" next year.
"It has real results, incredible success stories and I'm looking forward to being able to support these people on my radio shows."
Neneh Cherry's daughter, an unsigned rapper and a former backing singer for Pulp have all made the longlist for the BBC Music Sound Of 2016.