[One (M7) Q&A, Help & Troubleshooting] syserr0r:
My opinion of the HTC One (M7) Sound

I've been lurking for a while, never really posting but I couldn't find any solid information on this (I really wanted to know the DAC but oh well) so I'll post what I know now that I have my HTC One.

I have a pair of Shure SE210 in-ear headphones, V-Moda crossfade LPs over-ear (closed-back) headphones and a reasonable Hi-Fi and amp/receiver -- all of these are not el-cheapo pieces of kit so (while subjective) should provide pretty good audio quality.

I also have a pair of the Monster 'urbeats' that came with the first orders for the HTC One X and an X-mini II portable speaker.

I now have an HTC One. It is carrier branded to Vodafone. I am in the UK (so international variant of the HTC One).

This may also mean there is a volume limit applied to the headset (EU regulation) although I highly doubt this given the max comfortable volume I would listen to (see below) being well below the phone's maximum.

Disclaimer: They are not high-end, nor are they 'flat' studio headphones (typically they emphasise low/mid more) but they should be good enough to pick up any issues. For reference the Crossfade LPs should compare to top-end Monster/Beats.

Disclaimer: I am no audiophile. My ears are probably shot from listening to music too loud. My music collection is primarily 320kbps MP3 with some lossless (FLAC) mixed in. It is fairly varied but is typically dance/pop/rock/electro/club/house/drum'n'bass -- I'm not going to be listening to classical music and telling you how I can hear the conductors heartbeat.

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Loudspeakers

They are pretty loud

They are louder than the volume of conversation in a quiet room

The speakers are on the front of the phone, yet if I have it facing away from me the sound is not muffled nor significantly quieter

The sound quality is very good for a phone

Comparable to a small battery powered portable speaker (very close to my X-mini but lacking bass compared to the X-mini)

The lows are okay, but not that loud (if it was, it would sound horrible anyway)

The mids are good

The high is okay/good

There is no noticeable distortion at max volume with a source that doesn't clip

Headphones

They are loud

Too loud for me to listen to comfortably on max volume

Updated: For extended listening I would not set the phone volume to higher than

~40% on the Shure's

~50% on the urbeat's

~60% on the V-Moda's

There is no audible background hiss or buzz (HTC One X suffered from this)

There is no 'click'/electrical 'disconnect' when other sounds are played/audio is switched to speaker such as on SMS Notifications/Phone Call (HTC One x suffered from this)

There is no noticeable distortion at max volume with a source that doesn't clip

The quality compared to plugging my headphones into my AMP/Receiver is similar

The AMP/Receiver has its own EQ settings so I would expect it to sound slightly different anyway

The difference between Beats audio enhancement on/off is not as large as the HTC One X

I would estimate a 2~5% volume increase

A slight increase in the mids

Overall quite pleasant to have enabled

Still sounds pleasant if it is disabled (unlike HTC One X, HTC Desire which sounded very flat without some sort of EQ)

Update: My personal preference is now off (since my headphones emphasize bass a fair amount) and to adjust levels in an EQ on the app; rather than having the audio go through 2 EQs

Summary

It is the best sounding portable device I have ever owned

I have never owned nor used any Apple or Samsung phone so I cannot give a comparison to them

That said, the GSM Arena sound testing shows that the iPhone 4S, HTC One and Samsung S4 have almost identical sound quality (see below)

A lot of what I have written is subjective, and is therefore my opinion and not fact (although very little is in the audio world)

Reference: GSM Arena posted audio measurements for the (gsmarena.com/htc_one-review-912p7.php) HTC One and (gsmarena.com/gsmarena_lab_tests-review-751p4.php) some other phones (older review, of which the iPhone 4S was the best at the time) where the HTC One scored very highly, matching to all intents and purposes the iPhone 4S for quality. They also posted a (gsmarena.com/samsung_galaxy_s4_vs_htc_one-review-913p5.php) HTC One vs Samsung S4 review with the same tests where the Samsung S4 had almost identical results (slightly better stereo crosstalk) although the HTC One was significantly louder.

PS: If anyone can tell me what the DAC is and/or how it compares to other mobile phone DACs that would be great.

PPS: Sorry, I can't post the links to GSM Arena properly because I have less than 10 posts -- copy & paste above [or if a moderator can edit the post, that would be great]