Welcome to Android Central! Can you give more details? Did it abruptly stop? Were you having problems leading up to it? Is this for all sounds--media, ringtones, notifications? Do sounds work over wired or Bluetooth headphones? Have you tried a reboot? How about booting into Recovery and wiping the cache partition, which won't erase any personal data:
https://support.t-mobile.com/docs/DOC-14686