what happens if you press these buttons :
Workaround to Adjust Volume on Chromebooks When Using AIRSecure Test Mobile Secure Browser
You said “If I press the decrease the volume button the sound gets muted.”, but you don’t say what happens if you press increase volume…
On picture which you sent press :
Mute Mute (F8)
Decrease volume Lower the volume (F9)
Increase volume Raise the volume (F10)
Does it help?