I own an Alvarez Yari acoustic guitar and before I changed the strings it sounded good. Though given the fact that the strings are already old, I changed them and now it does not sound as good as before.
If you changed strings and bought a set of nylon strings in place of the steel ones, the sound coming from the nylon strings may sound “dull” in your ears compared to the steel strings. A technique used by some acoustic players is to place steel strings at the bottom part of their guitar (the last 3 or 4 strings) and use nylon strings on the top part. By doing this, the guitar will not screech when high notes are played.If you are using your strings for a long time, it is possible that the sound that you are hearing comes to you as weak because you are not hearing the overtones from the string (or the thing that gives each string their own “personality”).
It is also advisable to check the gauge of the strings. Thick strings will sound “fuller” or “duller” while the thinner one will have a “brighter” or “harsh” sound. Buying strings with a thinner gauge will give you a livelier sound.