Here are soft plastic rings that can be placed around dog tags and almost completely muffle the "jingling" sound. These "Quieters" measure 1.25" diameter and can be stretched to accomodate most dog tags. They are available in 8 colors: red, pink, aqua, blue, teal, magenta, yellow and black.
Price is $1.00 per quieter and there is a minimum order of 2, including shipping for USA residents. International shipping is extra.