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.
If you're flying United (Continental) from Newark, take into consideration that it's a hub, with loads of international flights from Buenos Aires to Tel Aviv. Lines can be longer and delays should be anticipated. Check-in requirements can vary drastically depending on your airport or destination. For instance, Delta requires that voyagers heading to Moscow and Istanbul check in at least three hours before departure, more than three times earlier than if you were going to Las Vegas. It's best to follow rules strictly.