Owned the M8 since release day, just picked up the 10 about a month ago, currently working on a review, so here are some of my notes:
As far as SQ through the headphone jack goes, the quality is noticeably better with a very low noise floor even with my most sensitive iems, the Shure SE215, Westone UM 50 Pro and Oriveti Primacy, it's barely noticeable if at all. The M8 by comparison had a lot of hiss with those earphones.
Output impedance is definitely lower as it sounds more inline with my Oppo HA-2 than the M8 does with particular earphones.
Volume is pretty similar between both, I don't own anything particularly hard to drive and I'm more of a low level listener so hiss is usually a bigger issue to me.
Overall, I'm really liking the headphone out on the 10, it has HTC's usual quality AMP with plenty of voltage for high impedance gear and also plenty of current for low impedance monitors but this time, the discrete DAC is of much higher quality (Likely the Aqstic DAC), reproducing songs with a lot more dynamic range, everything sounds a lot cleaner.
As for speakers, it's a mixed bag as others have stated. I do think that the full range driver on the bottom/side of the phone is of slightly higher quality than the drivers used on the M8 but the top tweeter is pretty average.
The bottom speaker is mostly dominant with the front speaker complimenting certain sounds. The 10 does have the immediacy of sound that the M8 produces through this setup, where purely bottom/rear firing phones can sound more diffuse, but it lacks the sense of depth produced by the stereo setup.
The 10 has more extension in either direction, treble in particular is much crisper though the lower midrange has less body. Bass notes are punchier through the 10 though the midrange sounds somewhat skewed due to the uneven speaker output. More detail and clarity on the 10's speakers are great for videos and vocals in general.
I think the sound quality of the 10's speakers is ever so slightly better but the placement is of course inferior. The M8 has an extra notch of volume but distorts slightly at the top two notches, the 10 does not, so in that sense, the usable volume range is higher, but overall volume doesn't get quite as loud. You'll notice a lot of internet reviews use the music preset, the movie preset actually produces a lot more volume but soundstage effects can make vocals sound more hollow.
If you use the phone speakers to listen to music, definitely check out this video detailing some eQ settings though Poweramp, you can also see my tweaked settings below sourced from this video, both do produce a much nicer sound than stock when used in conjunction with the Dolby music present. (
https://www.youtube.com/watch?v=UEQsxcoh3Uw)
Of note, 31Hz is reduced to minimum in order prevent distortion as phone speakers will never reach that low anyway. Be sure to assign this eQ to speakers and the flat eQ preset to headphones so Poweramp will automatically switch between to two when headphones are plugged/un-plugged.
Tweaked eQ settings:
Very insightful. 
