Hi,
If the tablets were purchased new in December 2016, they are still covered by the manufacturer's 12 month warranty.
Verify the purchase date on the sales receipt or proof of purchase document to ensure that the warranty period is still valid and then consult the warranty statement (found in the User Guide) that came with the tablets as to what you have to do to make a claim for a manufacturer's warranty repair or replacement.
If you can't find the warranty information, here is a link to the warranty section of the tablet's User Guide which may be of some help. (click on the Next page link at the bottom of the page in the link to view the rest of the details).
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The same thing happens with mine. The sound or volume only works when I plug in headphones or earbuds, or I cast a video. There's gotta be something wrong with it. I mean, I could take it to get it fixed or checked out, but I could also just get a new one and transfer everything.
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