Sound controls change the volume level, but not the actual volume

I've been having this issue intermittently - sometimes it just goes away, other times I have to restart.

When I adjust the sound with the volume keys (f11 and f12), the picture pops up, it says my volume is being changed, but the volume level does not actually change. If I go into system settings I can manually adjust the volume, but the volume buttons don't change anything. It's almost like my volume buttons and the volume indicator that pops up are completely detached from the actual volume of my MacBook Pro.

I have a mid-2014 13" MacBook Pro running Yosemitie 10.10.3

Thanks!