Kennel cough or something worse?

[Sumpet]

We had a health dog one day and the next coughing and hacking. His cough sounds dry like a honking goose. Took to vet he said could be kennel cough and put him on doxycycline but also said he heard a heart like murmur. The vet did say that the heart could have sounded like that cause of the dog not feeling well if it was kennel cough and/or being anxious. How did I have a vibrant healthy eating dog one day and a sick one the next. His appetite seems still to be ok. He just wants to eat anything else except his dry food at this time.