Hi recrutabrasil5,

Sorry to hear that you are having this issue!

Before taking any further steps, you may want to make sure that you are using a supported browser and clearing your cookies and cache, as described here: https://support.khanacademy.org/hc/en-us/articles/360029043712

If this does not fix the issue, could you please submit a support ticket so that we can further investigate the problem? Including information on what specific videos are not playing sound for you and what browser you are using are important to include in the ticket. 

Thanks!