I'd like to create a "muffled" effect, so it sounds like the music is coming from the next room over, and I can't figure out how to do it in kdenlive. Any help?

I'm using the latest version of Kdenlive on the latest build of Ubuntu Studio.

Have a look under Effects -> Audio -> M-R -> Multiband EQ. Just set all the frequency bands above 1250Hz to -70 gain or so.
Or just use the Low Pass Filter, it's simpler to use and probably does the same as the above if you set the "Cutoff Frequency" to something like 1250Hz.