Have you had a look at the DIY Projects section? There are numerous projects relating to electics and extending electical circuits. Also there are several projects and entire forum relating to sound proofing that may be of some use to you: http://www.diydoctor.org.uk/projects.htm
Hi if your thinking of running that much equipment in one room i suggest you will need a dedicated circuit from the fuse board to supply that . Unless everything is very low wattage then you will be ok using the existing ring main (providing you have a ring main )
Although you can add sockets there are also some 10 amp extension leads available with 8 and 10 outputs per unit and filters built in much depends on what your music equipment is but some times it is easier to have it all on one plug or switch. Much of the electronic equipment has current draw of few milli-amps and you really do need to look at it all and see what supply it requires to convert watts to ma divide but 230 so 20 watt unit = 27ma or 0.027 amps that's what my sky box uses. My old 14" TV 50W so 217ma with loads like this 20 items could still be less than 5 amp.
So before going to any great lengths do check how much power they do use.