Hi nhg-1234,
Thanks for using Apple Support Communities. We understand that music is stopping when you open WhatsApp. We certainly want to make sure you're able to enjoy your music, and we're happy to help.
If you haven't done so already, restart your device. A restart is a simple step that can resolve a variety of issues. See how here: Restart your iPhone
Also, make sure there are no pending app updates. You can see how here: How to manually update apps on your Apple device
If this behavior continues, reach out to WhatsApp support here: Contact WhatsApp
Thanks.