Hi @Hulksmash, I am sorry to hear about this, let me see if i can be of any help here.
Firstly i would suggest that you test the volume down key using the support application in order to see if the key itself functions correctly. In order to do this, find and tap Settings > Tap Support > Tap the Tests icon in the bottom of the menu > Volume down button.
If the volume key pass the test i suggest that you perform a software repair using a computer and Xperia Companion. Please note though that a software repair will erase all user data, so make sure to do a backup before repairing the software.
If none of these measures manages to improve the function of the volume down key I suggest that you contact your local support team in order to have the phone sent to an authorized service center. If you have any questions regarding the service, such as routines, warranty and consumer laws, these type of questions are best answered by your Local support team as well, as they vary between markets.