Before anyone goes crazy installing a mega-sound system in their Elio, your first consideration should be the acoustic environment. Remember this is designed from the outset to be an economy car - so it's not going to be hauling around a lot of expensive insulation. At present there seems to be NO insulation in the cabin, only a thin plastic shell between you and that noise-maker up front, and three tires droning away to either side and out back. You're going to start with a noisy little box, and no amount of kilobucks is going to overcome that. It's going to be LOUD in there. The fix is relatively cheap, but it is going to be fiddly.
I'd consider a complete insulation package as a first step - pull out any interior pieces you can, seats, any trim panels that you can safely remove, then trim and stick slabs of Dynamat everywhere you can reach, particularly to the firewall and floor - all the way back to cover the rear wheel housing. This MAY get you up to par with the cabin silence of most mid-line cars. Check any holes in the cabin and install close-fitting rubber bushings where wiring or mechanical linkages pass through the firewall and engine noise can enter. You may want to install a tach - after treating my boat in a similar manner, I found it was about the only way I could tell when the engine started.
Once you have most of the thrashy and rumbly bits silenced, your Elio may still have some sound leakage issues, particularly in the single-seals around the door, hatch, and lightweight windows. Luxury cars today often have triple door seals, even extra-thick windshields that positively seal the cabin from road noise. There's not much you can do with these, but by controlling most of the exterior noise from entering the cabin, you'll have a huge head start on building an entertainment system that will justify its expense.
