Things have been a bit slow whilst the suppliers and I resolved our differences on the sound proofing, however we have come to a mutually satisfying result and so I can start cracking on.
The first step is fixing the mistake I made with the specifications and sorting out the holes cut into the acoustic treatment.
Step one, make some 'plugs' with four layers of acoustic plasterboard, glue, leave under pressure to set:
Next add the new surface-mount electrical boxes, glue, leave under pressure:
Finally, slather everything with acoustic caulk and fill all gaps with the same:
All that x4. Painting will happen later to tidy things up a bit. They're bulky, boxy and stick out a lot, but they'll be much less obvious once the bass trap frames are in place and they've made a significant difference to the overall sound proofing. When it's not raining the room is now as quiet as the quietest room in my house. Not bad for a standalone, single-skin shed. :)
Speaking of bass traps, I got a bit of work done on the framework whilst waiting for glue to dry: