Music output from iPhone will drop when I access Facebook and get messages..Help?

[nikettle]

When I leave the Spotify app and access Facebook or I get a message, the output volume from my phone (when plugged in to an aux cable) will drop almost to half of the volume I had it at and won't return to normal unless I pause the song and press play again. Is this a bug with the Spotify app or is there a way I can bypass other apps/ sounds to prevent this problem?