How can I hide all the wires when I'm wiring in an already built room for a home theater?

wire moulding, any electrical supplier

Double sided velcro. It is a necessity in my job, where we have very high and very public client. Group the cables together and then use double sided velcro evenly spaced to hold the now 1 big cable together.

If you're really keen you can get these cable covers where you put all the cables together and then run this zip thing over the top.

Tape it to the corner of the walls between the wall and the floor with the same coloured duct (electrical) tape and if you cant match the colour just get the tape painted the same colour as the wall

My dad does that kind of things, what they do is make lil holes ( that no1 see) at the wall and passing the wires inside.

go for a satelite speaker system!

hide them under your baseboards-that's what I did

go through your attic