This one can be a bit tricky. First and foremost, try to plug in a pair of headphones you know work. If the headphones seem to be working all right, your speakers probably broke. See the repair guides for how to do this.
If the headphones also don't work, the problem may actually be the motherboard itself. This is a slightly more complex fix, but there's a repair guide for this too.
Both of the repair guides mentioned may be found on the device page, www.ifixit.com/Device/Nabi_Jr
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