When we did our Town Sedan, we used speakers in the front cowl section and hid them with "map pockets" and snaps; that way it creates the appearance of being relatively "period correct", but also allows for a more permanent sound base as well. The controls and amp are hidden under the front seat, but we also ran a charging cable and I-Pod connector up between the cushions on the front seat. The addition of a "splitter" also allows for additional "auxiliary" speakers to be run into the back seat area - again, not a permanent speaker set-up, but mobile as has been discussed above. The sedan is used mostly during the Winter months, so with the windows rolled up, its not difficult to get some nice music to come through - sort of a "surround sound" when auxiliary back speaker(s) are used. During the warmer months, an I-Pod and a portable speaker work fine in the roadster and phaeton - with open cars, I don't think that a more permanent speaker mounting would produce any significant benefit over a more portable speaker set-up. Good luck to you regardless of whatever route you choose.
Dick