How to talk to dragons

Dragons would have two types of vocalization: high and low. Low voices are slow to make but travels long distances. High voices are fast to make but travels short distances. Thus, they can call out to each other with a low, droning sound generated in their chest. These long-distance calls would be generic messages like "Stay out!" "Want to hunt together?" and "Lemme smash".

When they are close, they use the high voice generated entirely in an organ called the voicebox. The voicebox is very small compared to the dragon chest. It is probably located somewhere on the backs of their necks, to protect it against the elements, impact damage (A dragon's life is hard. Frontal assaults are common.), and from fire and smoke damage (which is why it's not in the throat of the fire-breathing dragons).

The voicebox has its own, little "second lung" to pump air through.
Now, how would humans talk to such dragons? They can make the high sounds by mouth, though they probably can't make them loud enough for dragons to hear, but the low sounds are just too low and loud for them. The answer is music instruments.

For low sounds, they can use giant organs powered by waterwheel. The dragon-talker would sit in front of a small keyboard (placed far from the organ tubes, to avoid sound damage to the dragon-talker), and each press activates a very low, loud tune. Playing the right combination of keys in the correct order can make the organ "talk".

For high sounds, the human can use a megaphone.