I don't know XNA, but I can give you a generic answer:
What you want is a "low-pass filter" which is a kind of EQ, or equalizer, sometimes simply called an "audio filter". If the system you are using does not have one, you can use a moving average filter: so each sample output will be the average of the previous N inputs (the bigger N is the more "muffling" you will get, but past three or four, you'll be doing a lot of work for not much return).
If the moving average filter isn't sufficient (and it probably isn't), the standard approach is the "digital biquad". I describe how to write your own in this blog entry:
(I describe a bell filter, but a low-pass is simpler.)