a speaker box will heat up like a motherfucker with a living plant and lights. if you go to your local department store (wal-mart, lowes, home depot...) they will most likely carry CFLs that are in a range of 40 to 60 watts putting out 2500 to 3500 lumens a piece. Using one of these for veg is sufficient if you are planning on growing a small plant which you would have to be doing if you are growing in a speaker box. I would highly suggest painting the inside white or purchasing some reflective mylar from a department or hardware store. I use one 45 watt cfl in a custom built cabinet and i still have problems with heat and that is with a 115v exhaust fan and the doors open. growing in something as small as a speaker box is going to be a challenge especially when you start to flower. this is when the plant needs more light and the plant will be creating more heat so adding light would not be an option. I would suggest something bigger,, i constructed my veg box out of an old malamine bookshelf.. equiped with a door and venting... here are some pics to give you an idea of my veg box.
the dimensions of this box are 16w x 11d x 32h
note: this is not the box i use to flower,, that is a closet/cabinet with dimensions of 3.5w x 1.5d x 5h... that is where i use my 70 watt HPS with vertical tube flourescants @ 30watts each on both sides of the plant and the temp is maxed out in that as well
p.s. for vegging with cfl go with a colour temperature of aprox 6500 k