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.
Delta and most of the other US network carriers have checked in bags to the final destinations even when the passengers have had separate tickets. This practice is coming to an end on Delta starting on January 15, 2013. Delta has issued the following statement, although it is not publicly available on Delta.com.