The behavior of a sound wave is affected by the density of the medium, the temperature of the medium and the pressure of the medium if it is a gas. Motion of the medium independent of the sound wave may also affect the way sound propagates.
Yes, you may check-in for your Delta Air Lines (DL) flight with your mobile device starting 24 hours and ending 60 minutes before the scheduled departure time (depending on your departure city and destination) to receive a mobile boarding pass on your mobile device.
The Delta flight is cheaper. But check a bag at a cost of $25 for the first checked bag each way on Delta, and your total fare climbs to $256 -- more than the cost of the Southwest flight, which includes free first and second checked bags. Add a second checked bag for $35 each way on the Delta flight, and you'll pay a whopping $326.