How does altitude affect sound?
The answer to your question has a lot to do with how waves generated by a moving object propagate. You can see a full explanation here but the short answer is the Doppler Effect which causes pitch to change as the object moves buy you. 

Here is a nice explanation

Edit:

My knowledge is a bit rusty but basically you are talking about a mechanical wave propagating through a fluid (the atmosphere) which will absorb some of the energy (eventually all of it). I would also thing that as the speed of sound changes the frequency of the wave would change much like the changes light has when it crosses different mediums.