Do you usually use the speaker, rather than headphones? Do you also have your speakers on really high? It could be that it blew out, but I'm hoping (and I'm sure you are as well) that that isn't the case.
The right speaker on my Lumia 822 stopped working, for some strange reason. It could be the phone, it could be a software malfunction. Check for phone updates. If nothing else works, go to Verizon or ship it to Nokia as long as it fits under the 1-year limited warranty. Best of luck! *fingers crossed*