Obscure effects on the distance a sound will travel

This is called the inverse square law. It is often stated "The loudness decreases 6 dB for each doubling of the distance".

Sometimes sound will reach a lot farther than the inverse square law implies. This happens when the energy of the sound is focused somehow rather than distributed evenly on an expanding sphere. Some common situations:

Directional loudspeakers
It is possible to build a speaker so that the sound is projected mostly in one direction. In fact, it's difficult to build a speaker that doesn't do this to some extent. Directionality is always more pronounced at high frequency than low.
Reflective support
The back wall of a bandshell and the ceiling of a stage are designed to bounce sound into the audience. The mountains around Bozeman Montana do the same for train horns.
Atmospheric effects
Since the speed of sound differs according to air temperature, sound traveling along a boundary between warm and cold air tends to get bent. Well, bent may not be the right word (refracted is) but what happens is the sound is diverted into the cold region. If you have a temperature inversion, with warm air above cold air, sound will be refracted back to the ground, as people who live on a coastline will tell you.