Why does my guitar sound quieter after years of play?

Before you assume your guitar is dying of old age, there's a much more boring and fixable explanation waiting in most cases: dead strings, humidity drift, or a setup that's slowly drifted out of spec over the years.

The usual suspects, in order of likelihood

• Old strings. This is genuinely the most common cause of a guitar sounding flat and lifeless over time, and it's the cheapest to rule out. Dead strings lose brightness and sustain gradually, which means you might not notice the decline until you play a genuinely fresh set and hear the difference.
• Humidity changes. A top that's too dry or too damp doesn't vibrate as efficiently as one sitting in a healthy range, and this can genuinely affect perceived volume and resonance, not just structural health.
• Action creep. If action has slowly risen over the years, you may unconsciously be pressing harder or playing less precisely to compensate, which changes your tone without you realizing the cause.
• Setup drift generally. Truss rod relief, saddle height, nut condition — small changes compound over years and can add up to a guitar that plays and sounds noticeably different than it did when new.

Is it ever the wood actually "wearing out"?

Some players and builders believe wood and finish continue to change tonally over years of vibration (sometimes called "playing in"), generally in a positive direction rather than negative. A genuine decline in volume or tone is far more often explained by the maintainable factors above than by the guitar's wood somehow degrading.

What to actually do

Change the strings first — it's the cheapest test with the highest odds of being the answer. If that doesn't solve it, a proper setup check (relief, action, humidity) is the next step before assuming anything more serious is going on.