This thread in the Audio forum is about an Android-specific problem, so I thought I would link it over here in case any Android experts have a solution:
We are experiencing this problem with the most recent 5.4 version of Unity. Something was changed in the way Unity manages the audiofocus and there is no way to configure that. Is it related to the addition of the “Mute other audio sources” button to the android player configuration? Before the recent changes everything worked well.