Can't unmute headphone volume

[Tb311]

Anyone else run into this issue? I usually have my phone on vibrate mode at work and sometimes when I plug in my headphones to listen to music I can't unmute the volume. I have also tried switching the sound mode from vibrate to normal first and then plugging in my headphones and it still gets stuck sometimes. When I try to increase the volume I just get the volume window pop up with a line across the speaker icon and the volume won't increase. The only solution I have found so far was to open the task manager and close all tasks and then I get audio.