Actually, I can think of many situations where speaker volume could do with a boost. Older iPhones in particular, the ones with just a single speaker, don't get very loud.
If only you could wave a magic wand and solve that problem. Eh, forget the wand, just use your finger instead. (This hack works not only in Apple's Music app, but Pandora and Spotify.)
Step 1: Tap the Settings icon, then scroll down and tap Music.
Step 2: Tap the EQ option.
Step 3: Tap Late Night.
Hey presto! Your speaker should get noticeably louder. This works best if you're already playing some audio when you venture into Settings and make the EQ selection. Music in particular seems to get the biggest boost, though it can vary depending on the kind of music.
I also noticed a slight increase in spoken-word volume, specifically podcasts played in the Overcast app. This was on an iPhone 6s Plus ($270 at eBay). I wasn't able to test an iPhone 7 ($194 at Amazon), which is blessed with stereo speakers that are already louder. If you try this, please share your results in the comments!
This is, of course, the simple (yet rather surprising) outcome of fiddling with equalizer settings. I suspect most users forget those settings are there at all, and while many of the choices do impact volume level, it's only Late Night that gives a big boost.
Now, do you really want that boost? You may experience some distortion, especially if you've got the volume cranked all the way to maximum.
But for those times when you find yourself wishing that your iPhone's maximum volume wasn't actually the maximum, give this trick a try!