Missing volume indicator
Missing volume slider
Check your connectors
If there's debris in the Lightning connector or 30-pin dock connector, your iPhone might think there's an accessory connected. Try cleaning the Lightning or dock connector on the bottom of your iPhone with a clean, dry, soft brush, then test the volume again.
Lightning connectors
iPhone 5 and later have a Lightning connector.
30-pin dock connectors
iPhone 4s and earlier have a 30-pin dock connector.
If you still need help
If you still don't see the volume slider or volume indicator, you might need to set up a repair service for your device.