I tried messing with the Spotify EQ and with the phone's sound settings, nothing.
I had a Note 3 until this past weekend and lost it. I got my new Note in the mail and redownloaded all my Spotify songs. However, now when I listen to my music through headphones or auxiliary jack, the songs start out at full, normal volume for a split second and then sound muffled. It's consistent through all my music. A half second of normal sound and then its like a filter is put on and it sounds much worse and lower volume.