Hello,
there is one thing that is very easy to fix but wrong since years:
In the right side bar a disc icon is shown instead of the volume up icon.
This is only because of wrong CSS classes. Replace “fa fa-save” with “fa volume-icon fa-volume-up” and everything is fine.
As all icons in the right bar, the volume icon is correctly shown on the left side. The save button is to make the change permanent (save). 
There are no volume up/down buttons. To change the volume you have to use the slider in between.
Three weeks without any answer from a developer, and even without fixing despite simple solution. Unfortunately this is not the first time, so I cancelled to be a site supporter.
Hm? Have you not seen the answer by Flovo?
He is quite correct, the volume icon is supposed to be only on the left. You adjust the level by the slider bar and if you want the change permanent you click the save icon (otherwise the change reverts to previous levels after the current session.
I guess maybe you would prefer a different approach or design, maybe a better way indeed exists, but as it is not broken the way it is now, change of such a small thing is very low priority.
Thank you for your answer. I thought it was a joke, and because there is also a CSS class and an icon for volume up in that context, I thought the disc is wrong. But I’ve tested it now and it works, as you have described.
So it works as intended. Maybe it’s more clear to have a volume up and a disc icon on the right side? But I must admit that it maybe looks bad.