A muffled tone is one of the more frequent issues players run into, and the cause can lie in the environment, the soundboard, or the contact between components.
High ambient humidity is the most common cause. Once the soundboard absorbs moisture it gains weight, vibration travels less efficiently, and the sound loses its openness.
Poor contact between bridge and soundboard affects tone as well. When the foot does not sit fully flush, the transfer of vibration is compromised and the sound tends to feel thin or closed.
Aged strings will also degrade the tone. As elasticity falls and the outer winding loosens, clarity drops noticeably — replacing the whole set often brings a clear improvement at that point.
Check in the order of ambient humidity, bridge condition, and string age. In most cases this will pin down the specific cause.