What are the rules for how far sound can travel in 5E?
The only rule I can find is the DMG description on page 105:
A dungeon's enclosed geography helps channel sound. The groaning creak of an opening door can echo down hundreds of feet of passageway. Louder noises such as the clanging hammers of a forge or the din of battle can reverberate through an entire dungeon. Many creatures that live underground use such sounds as a way of locating prey, or go on alert at any sound of an adventuring party's intrusion.