I've had instances where I want to share videos on my Xperia Z Ultra with friends in a louder environment and we're barely able to hear anything at all. It does sadden me that my monsterous phones loudspeaker can be out performed by something like an iPhone 5. Of course I knew to expect this going into my purchase, and was fine with it as I'll be upgrading this year.
Anyways, based on the one video we've seen so far, it's impressive how comparable the Z2 speakers are to the HTC One considering the phone is waterproof.
BTW, it's interesting to note the newly announced Xperia E1 has a loudspeaker capable of over 100 decibles. Imagine if Sony decided to build a non-waterproof phone and put 2 of those babies on the front for stereo sound! Just to give an idea of what 100dB means, here are some examples I found from a quick google:
Jet take-off (at 305 meters), use of outboard motor, power lawn mower, motorcycle, farm tractor, jackhammer, garbage truck. Boeing 707 or DC-8 aircraft at one nautical mile (6080 ft) before landing (106 dB); jet flyover at 1000 feet (103 dB); Bell J-2A helicopter at 100 ft (100 dB).
8 times as loud as 70 dB. Serious damage possible in 8 hr exposure