Over the last 10 years, we have recorded hundreds of thousands of foreign words with dozens of native speakers. Sometimes a word you add to the system won't have a sound file, especially if it's a longer sentence which we haven't encountered before. Where possible, we will use an electronic voice until a recording session with a native speaker can take place. Our recording schedule varies based on the amount of times a word has been used by students, and whether we can find a native speaker available to lend us their voice. In particular, we don't record words until they've been answered by at least ten different students in other modes.
If you have questions about a specific set of words that you've added, please contact firstname.lastname@example.org and our Content Team will be able to give you more information.