I don't really understand the question either...but working with what I can gather from it,
-sharing the monitor requires either unplugging the vga/dvi cable from the dell and hooking it up to the macbook via a mini dvi->vga/dvi cable, or, if the monitor has 2 inputs, hooking up both computers and using the input selection button on the monitor.
-sharing the speakers: unplug from the dell, unless you're referring to a real stereo with multiple inputs.
-by 'hooking up the tower to the mb', i'm going to modify your question to mean 'how do i share files between the 2 computers'. Set up a network, which i assume you already have if you plan on using 2 computers at once (network means you have a router (wired or wireless) or switch etc. that both computers are attached to), then share files between the systems. There are plenty of tutorials on this all over the place. you can also share printers easily this way.
Overall, it's not hard to use 2 systems, you just can't magically make them somehow share all your peripherals. sharing the monitor if it has multiple inputs is a good and easy move, but with things like audio and multiple computers you're better off with a real stereo with multiple inputs and real sound.