i have a ZTE Z998. i can receive calls and hear the ring tone. but the notifications for email, facebook, texts, or voicemail will only vibrate and no sound. i've changed the notification sound, and still nothing. i've got all the sounds turned up, and notifications turned on with first Celesta, now fairyland tones. nothing is muted in settings.
Check the settings of the app itself. Sound notification may be disabled in there.