Why is there no sound for this sentence? A set of possible answers.
When sentences are first added (and we add new ones all the time), it can take a day or two for sound to be uploaded. This is much faster than would be the case if we had a human recording the sentences, but it is not instantaneous.
There may be something up with the sound on your machine. Making sure the sound is turned on is always a helpful step.
If worse comes to worse, check out Duolingo's support resources, since we can do nothing about it through reports. Head on over to the Duolingo support page (duolingo.com/help) and check out these steps: