There are a few effects that come in place as rubber gets stretched. Mechanically the speed of propagation of a wave in a string (v) is proportional to the square root of the tension of the string (F) (http://en.wikipedia.org/wiki/Vibrating_string), this affects the interaction time for internal friction and could result in a overall reduction.
A more detailed explanation should require a look on the chemical/physical interactions that take place while the material is compressed or stretched, witch affects molecule alignment (compression stretches and aligns too) and overall Van der Waals interactions.
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