I've added a few mp3's from my local collection into Spotify to sync to my phone (Android, 4.2.2). The problem is, when I go to play the synced files from my phone, only the right channel audio plays. If I have headphones on, only the right ear works, if I use an aux cable in my car only the right side speakers work.
I've tried multiple headphones and aux cables, as well as bluetooth audio in my car, they all yield the same result. The mp3's played on my desktop version of Spotify (OS X) play full stereo audio.
Both variants of Spotify are up to date. Streaming a song from the Spotify catalog or playing one synced from Spotify plays both channels, I have streaming and syncing set both to Extreme Quality.