A great example of a descending octave can be heard in the first two notes of Jackson Browne's classic "Doctor My Eyes".
https://www.youtube.com/watch?v=qKGTaplzmV4&list=PLpLh6jTfTvRjPCI_bhx1Sbq-t-ktfuHHR&index=4

Bon Iver's song Blood Bank is a great example of a descending Major 2nd. The first two notes of the song are how I remember this specific interval!
https://www.youtube.com/watch?v=bpFUc8ABDMQ