WhatsApp Turns Down Volume in iOS 9
In other words, sound coming out of your iOS device stops. This problem first appeared on devices running iOS 9 firmware.
You can replicate this issue.
Play some music and open the WhatsApp Messenger. Then, from WhatsApp, tap on the camera icon to click photos or add photos through the camera roll…
Music playing in the background mutes.
Volume also automatically turns off whenever you send a picture, audio, or video through the WhatsApp Messenger as posted here:
At this point, there is no fix available. Of course, you can downgrade from iOS 9 to iOS 8 if this sound problem bothers you…
But we recommend you to wait for a new WhatsApp update that is optimized for iOS 9.
Post will be updated when a relevant solution is found.
— iOS 9 Updates / News (@ios9apple) September 19, 2015