Notification display/sound no longer synchronized

For the past few days, the sound indicating a new message in the notification center hasn't been synched with the display -- sound alerting me to the display will occur well after screen has gone dark again (by at least a minute). I have a 4g and I'm running 5.1 (updated yesterday but the problems preceded the update), and I haven't changed any settings recently. If resetting the phone is the answer, do I reset settings or network or ... what? Haven't done that before. Appreciate any insights/suggestions.