Can metal framed single pane windows be sandwiched together to make an effective double pane

I've used the plastic window stuff before (literally just finished putting up some a few minutes ago) and it works quite well. Another option to look into is storm windows, they go a long way to helping keep a house warmer. You can purchase storm windows in standard sizes at the usual hardware stores, or order custom fit ones if they don't have the ones you need.

There are also weatherstripping products you can get for your windows. Wired currently has a decent overview of what your options are.

Single glazing with storm windows absorbs sound reasonably well, I would not expect a huge decrease in sound level with triple glazing, but certainly not an increase. Single glazing alone is another story, it's sound absorption is terrible.

The increased sound is certainly due to the frames and adjacent construction. While there is often a correlation between thermal loss and sound transmission, sound is much trickier to manage well. It is sometimes counter intuitive. The construction joints must be carefully detailed. The general concept is to avoid any solid connection between inner and outer surfaces. Any connections should be via resilient, absorbent materials. Typically, you want to fill any voids with sound insulation, though in some cases this has been worse than a simple air gap.

Also consider exterior sound attenuators. Walls, fences, trellises, large shrubs, trees, berms all will help attenuate sound, but when you consider cost vs. benefits, such measures need to be carefully considered.