For cold symptom relief, you might want to try adding a bit of whiskey to your miso soup. The salty broth and heat are the equivalent of chicken soup and the seaweed and miso are a source of several B complex vitamins if I remember right. In Japan, traditionally, hot Sake was added to help with scratchy throats, but more recently has been supplanted by whiskey. It's a bit like adding the same to chicken soup in its effect.

I'm not terribly familiar with miso in the refrigeration section. Normally, I buy the packaged or tinned miso paste that only needs refrigeration after you open it. While there are a number of kinds of Miso other than the Shiro (white) and Aka(red) miso types usually found in stores, I really don't know if any of the variants is a lower sodium variety.

Miso is made through a variety of methods to ferment a 'beer' of rice, barley and ground soybeans with brine. The active fermanting agent is called 'koyi' (sp?)
Miso is used for a wide variety of things in Japanese cooking, it is a pickling spice in Misozuke, an ingredient in sweet glaze in mochidango, and mixed with udon noodles, ramen or spread on hot rice to make an inexpensive meal.

If you're dead set on lowering the sodium content, you might try diluting the miso with some mochi flour and ground edamame (soybean) which should produce a soup that is a bit less salty.