I know Apple claims 25% louder with better bass- that’s pretty accurate. The top (earpiece) speaker on the iPhone 7 & 7 plus was never equal in sound volume or clarity to the bottom speaker. Apple mearly fixed this by replacing the top (earpiece) speaker on the 8 series with a higher quality one (almost) equal to the bottom one. I’m happy with the change and it’s one of the main reasons I upgraded. I also had a plus model & a regular 8 and compared the two. Really there’s no difference so I went with the regular: easier to handle day-to-day even though I have XL hands.
It's actually 25% louder and added bass according to Apple. And it is a big difference, given that the stereo speakers are fairly loud with the iPhone 7. So the increase is certainly notable with the iPhone 8.