Tedious as hell, but yes it would work. Leave out the duct tape, though. As long as your speaker wires are insulated, you don't need it. One layer is enough as long as there are no breaks/gaps in it. If you do go with multiple layers so as to make sure there are no gaps, put them right on top of each other. The whole idea is to create a single continuous conductive layer. Any outside interference will couple with this layer and not with your signal lines. Also, to ensure success you'd want the foil around each pair of conductors, one "plus", one "minus" per channel. In other words, generally no go on the "single ground wire" idea.
Theoretically, I guess, as far as outside interference goes, you COULD go with a common return (what you called a "ground wire") with a single shield around the entire snake. The the common return would need to be inside the shield, of course. This would prevent outside interference, but I think you may get unacceptable levels of cross talk between the channels as the signal wires couple (interfere) with each other. Then again you may not, I don't know for sure. I think you WOULD probably get cross talk, but if you're ONLY going to use this snake for micing a drum kit, you're going to have some cross talk anyway, from all the mics being live and not isolated, so maybe that's ok.
Final two thoughts:
1 - I am NOT recommending this per se, and offer no guarantees it would work, only the above comments as to what i THINK.
2 - If you do try it, be sure to post back how you make out, as I'd be really curious to know
J
PS - this may be really obvious, or you may already know, but the shield is the reason mic connectors have three pins: One for th "plus" signal), one for "minus" signal, and one for the shield. This would be called a balanced cable. With an unbalanced shielded cable, the shield has to do double duty, acting as a shield and as a signal conductor. If the electronics are designed for this, it theoretically will work just fine. Most mic preamps however expect a separate shield conductor so doubling them up can cause problems in many situations. Plain speaker wire is an unshielded cable. The conductors run side by side, so there is no shielding effect at all.
