For reference, the communication music is known as "Signals."
Listen carefully for musical Shout Outs to Jaws (when the mothership is communicating with the Devils Tower base) and Pinocchio (when Roy is being led into the ship).
The "voice" of the mothership was done by Jim Self, a legendary tuba player in his own right. Spielberg used him because the skill and difficulty of playing the tuba gave the mothership a "human" characteristic.
California Doubling: Mobile, Alabama, stands in for Muncie, Indiana. Additionally, nearby Bay Minette, Alabama, stands in for Moorcroft, Wyoming. Some scenes were filmed at the real Devil's Tower, though the climax was, of course, filmed on a sound stage... an aircraft hangar in Mobile, Alabama. The Ohio state line was filmed on the approach to the Vincent Thomas Bridge, in Long Beach California.
Everyone Is Jesus in Purgatory: The film can be interpreted as a metaphor for a spiritual awakening and journey: An ordinary man (Roy Neary) has an experience with a higher power (encounters UFOs one night on the highway) that completely changes his life (becomes obsessed with UFOs to the detriment of his family and social life) and is subsequently compelled by this higher power (through a mentally implanted image) to make a pilgrimage to a predetermined location (travels to Devil's Tower, Wyoming) where he communes with the higher power (takes part in First Contact with the aliens) and is ascended into the heavens (departs with the aliens after being chosen to join them). Roy getting sunburned by the UFOs during his initial encounter marks his symbolic baptism from normal everyman to alien contactee.
Harsher in Hindsight: Six years after Close Encounters was released, Colorado suffered a real toxic-gas spill following a train crash that forced an emergency evacuation, much like the cover story that was used in the film to justify evacuating the Devil's Tower area.
Hilarious in Hindsight: The humans who enter the mothership at the end are dressed in red jumpsuits and sunglasses, which in hindsight look like cheap versions of the aliens' outfits from the original V miniseries (in which the aliens did not turn out to be cuddly, friendly greys).