Pokémon GO disables or reduces the volume of background music

RESOLVED!
Last Updated: 11/02/2017
Update: This issue is now resolved in Pokémon GO version 0.81.1 for Android and 1.51.1 for iOS. 

We are aware of a bug in the development platform that disables/reduces the volume of music or podcast apps. This issue occurs even when Pokémon GO's music and sound is disabled in the Settings menu.