The Nokia 6 sports built-in dual smart amplifiers (TFA9891) that according to the press release, deliver a 6dB louder sound than a regular amp for clearer sound with more prominent bass. The phone also uses Dolby Atmos that simulates surround sound to make it sound like it moves within a three-dimensional space. So in stereo, you listen to audio coming from two channels. With surround sound, you are still limited to the sound coming from a specific position in space where the speakers are located, but in Dolby Atmos, the sound seems to just float around you from all directions and sound richer, clearer and we greater depth.
So the Nokia 6 combines a powerful power amplifier chip alongside dual speaker system (one speaker at the top and one at the bottom) to provide users with an exceptional audio listening experience, whether your are listening to music or watching videos. Nowadays were many people use their phones to listen to music, play games and watch movies, it's now important then ever to have a high-fidelity sound that can further enhance those rich experiences.
Dolby Atmos on phones works with any headphones or earbuds and it's bringing immersive multidimensional sound experience for mobile owners, not just for within your living room. Of course it won't offer the same sound experience compared to having several dedicated speakers.
The Nokia 6 features the same smart audio amplifier as the Google Pixel XL (source: ifixit.com), but according to the Nokia 6 press release, there are dual amplifiers rather than one, and this is why I believe HMD has mentioned that those dual amplifiers deliver 6dB louder sound than a regular amp.
For more information about Dolby Atmos, visit this page on dolby.com and for the NXP TFA9891UK Smart Audio Amplifier visit nxp.com.