I wondered why you made your work more difficult by building the desk first? Cladding the walls with it in place will be loads more difficult? Nice job though! Keep in mind that all your efforts keeping sound in and out will be compromised by the smallest gaps. Cutting around obstacles is always the place where you get leaks. I put a smoke machine in my last one, filled the room up and then looked for leaks, and found quite a few!
Edit
The material you are using on the walls - not sure of the US term. It's not got enough mass to stop sound passing through it. Horrible stuff with loads of air gaps and voids. Ply, MDF and gypsum based sheeting (12mm at least) work well. That boarding is not a good choice.