Why sound is too low?

My iphone has a problem. It's speaker sound is so low. Like when I make phone call and make video from original camera, then sound will be good. But when I make whatsApp call or call from any other social app it makes a very low sound same is the case with voice messages and voice memo. It is not muted, it's just too low. Siri is good, Siri works good, cellular phone calls are good and original camera video sounds are good, also I can hear others properly. But when I make snapchat video or send a voice message on snapchat, whatsApp, instagram or make call on all of these apps then sound will be too low. What problem is that? And what can be the possible solution?