All these sounds are codes of communication. Sometimes they do one short yelp or bark.
Dolphins can do one long whistle or one short choppy whistle.
Yes, these are codes because they understand what they are saying, but we do not understand what they are really saying. You could do an experiment yourself with a pet at home. Before, read about verbal codes and ciphers. We don't understand what they are saying. We also don't understand why our pets are saying when they bark, meow, growl, etc.