iOS 7 has text-to-speech in 26 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 3.7 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 36 voices, so you can hear the difference.
Below are the instructions for iOS 7, there’s a different page for iOS 8.
In Settings: tap General.
Tap Speak Selection.
Switch it on.
Go to the list of Voices.
Pick the language & voice.
Switch on 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.)