Q: Volume full blast when muted

I have a Late 2013 Mac Pro 3.7 that I use at work. I run OS X 10.9.5 on it. I listen to Spotify most days through a pair of headphones most days. My usual listening volume with my headphones on is 1/4 notch to 1 notch. Sometimes if I mute the volume and put the computer to sleep, when I wake it up and start Spotify playing again, the sound will come through the headphones at maximum volume, nearly deafening me, even though the system's volume level is muted. Does anyone know what may cause this, and if there is a fix for it other than readjusting the volume every time I wake up my computer?