You can hear the paragraph
"Please call Stella. Ask her to bring these things with her from the store: six spoons of fresh snow peas, five thick slabs of blue cheese, and maybe a snack for her brother Bob. We also need a small plastic snake and a big toy frog for the kids. She can scoop these things into three red bags, and we will go meet her Wednesday at the train station"
spoken 1300 times on an online accent archive, which has been set up by the George Mason University in Fairfax, Virginia. The idea of the archive is to have a collection of as many speech accents as possible from as broad a range of language backgrounds as possible -- native speakers, people who learnt to speak English as an adult and those who learnt English as a second language early in life. The paragraph was specially composed, as it contains most of the sounds of Standard American English.
The archive seems to confirm the generally held belief among linguists that there is a cut-off point, after which it is hard to learn a language without having a 'foreign' accent. Someone who learnt English at the age of 11 then spent the next twenty years living in the USA will have more of a 'foreign' accent than someone who learnt English at the age of four, spent the next five years living in the USA, then moved away. This second person is more likely to sound like a native English speaker.