I have both and the 10 is not even close; however, the sound through Bluetooth and the headphone jack are stellar. The bass is full and rich while the midrange and upper end are detailed and warm, and that's through Bluetooth. The headphone jack is all of that times 5. It's really that good!
If you're looking for the phone speaker to be loud, I think you'll be disappointed when compared to the 6P. But, if you turn the 6p volume up to the point right before distortion starts, then they are comparable in volume and the HTC 10 blows it away with sound fidelity. For me, I'm most impressed with audio through Bluetooth as that's how I connect when I'm in the gym. You don't do all of the sound profile stuff when you're connected with Bluetooth but it absolutely blows the 6p away. When you're connected with wired headphones, that's where the 6p becomes a real laugher. It's almost impossible to listen to any kind of music on my 6p now.
Posted via the Android Central App