Hey there Sunnydays98,
Thanks for reaching out to Apple Support Communities. I understand audio from certain apps stop when you lock your iPhone. This did not happen prior to iOS 13. I’ll be happy to help.
First, make sure that you’re updated to iOS 13.2.2 via Settings > General > Software update. This update addresses issues with apps in the background and may help restore previous behavior:
Update your iPhone, iPad, or iPod touch
If this behavior persists, force quit the apps, restart your iPhone, and test them out:
How to force an app to close on your iPhone, iPad, or iPod touch
Restart your iPhone, iPad, or iPod touch
If that doesn’t help, delete the apps in question, restart your iPhone, and install the apps. Test again and see if this resolves the issue:
How to delete apps on your iPhone, iPad, and iPod touch
Redownload apps, music, movies, TV shows, and books
If not, you an reach out to Apple Support at this link so they can continue to look into this with you:
Contact Apple Support
Cheers