iOS 8 has text-to-speech in 27 different languages, but Apple didn’t include the high-quality voices like you would have on a Mac. With good reason, because including all those Enhanced Quality voices would make the system 4 GB heavier.
The HQ voices have to be downloaded in your phone’s Settings app. But it’s easy. Just follow the steps below.
I made another post with samples of both qualities for all 37 voices, so you can hear the difference.
Below are the instructions for iOS 8, it’s a bit different on iOS 7.
In Settings: tap General.
Switch on Speak Selection.
Go to the list of Voices.
Pick the language & voice.
Tap Enhanced Quality…
…and wait until the download finishes.
Note that the enhanced voice might not immediately be active. It might take a minute or so before it kicks in, but you’ll notice the difference when it does.
Extra advantage: by switching on Speak Selection you can now have your phone read out selected text. (An extra ‘Speak’ option will appear in the copy/paste menu.)