Did the sound ever sound unmuffled or is this a recent issue?
I would recommend checking your signal path and make certain there are no "enhancements" made to manage the sound. So nothing like sound leveling to make songs sound the same volume or enhancements for clearer sound or to sound like a certain club or concert. You'll need to check the software playing the tracks, the speakers and any other items in your signal path chain. Also no internal equalizers or bass enhancements or mod enhancements. Get everything to run with as few of the enhancements a s possible, then you can tweak things from there.
Also, what quality tracks are being played? Are they MP3s, FLAC, WAV and what is the resolution of the tracks. The better the equipment you have, the better the signals that should be fed to them.
As others have mentioned you can get relatively inexpensive but good quality receivers that have built in room equalization that is applied based on measurements to the listening position often with an included microphone for the receiver.