I will describe the issue as I have experienced it a couple of times, with as many details as I can think of.
Nokia Lumia 920, WP8.1 Dev Preview, Spotify Premium, no matter if online or offline mode, custom notification sounds for other apps.
On Spotify startup, if a notification sound is played (e.g. a new message, email or comment arrives for any installed app and the phone plays a sound notification), the notification sound seems to stall and crack (as if resources are insufficient for the phone to play the sound AND start Spotify at the same time).
That would be bearable, but after such an issue, my phone loses its stereo - at least on wired headphones - and sound comes mostly out of the left headphone. The issue is only resolved by rebooting the phone.
Could be a sound driver issue, or incompatibility.