diff --git a/dev-journal.md b/dev-journal.md
index 913b86e..e9f52c7 100644
--- a/dev-journal.md
+++ b/dev-journal.md
@@ -72,4 +72,30 @@
 
 ---
 
+---
+
+### Session 4 - 2026-08-20 (Morning)
+**Status:** ✅ Sound Effects System Added
+
+#### What was done:
+- Created complete audio system using Web Audio API (no external assets needed)
+- Added click sound effect (rising C-major arpeggio on each harvest)
+- Added purchase sound effect (descending chime when buying buildings)
+- Added achievement fanfare (ascending chord progression on unlock)
+- Integrated audio into game.js, achievements.js, and index.html
+- Audio context auto-initializes on game start; browsers handle suspension automatically
+
+#### Files modified/created:
+- `js/audio.js` - New file with Audio system (click, purchase, achievement sounds via Web Audio API)
+- `js/game.js` - Added Audio.init() in init(), Audio.playClick() in harvest(), Audio.playPurchase() in buyBuilding()
+- `js/achievements.js` - Added Audio.playAchievement() on achievement unlock
+- `index.html` - Added audio.js script tag
+
+#### Next goals:
+- Mobile touch optimizations (tap feedback, responsive layout)
+- Prestige/reset system with bonuses
+- Consider adding more sound variations or music
+
+---
+
 *Journal auto-updated by Hermes Agent cron job (every 8 hours)*
diff --git a/index.html b/index.html
index 6f6d7aa..ef6a530 100644
--- a/index.html
+++ b/index.html
@@ -103,6 +103,7 @@
     <script src="js/ui.js"></script>
     <script src="js/stats.js"></script>
     <script src="js/particles.js"></script>
+    <script src="js/audio.js"></script>
     <script src="js/main.js"></script>
 </body>
 </html>
diff --git a/js/achievements.js b/js/achievements.js
index 8fad98d..d88279b 100644
--- a/js/achievements.js
+++ b/js/achievements.js
@@ -164,7 +164,10 @@ const Achievements = {
             if (achievement.condition(Game.state)) {
                 Game.state.achievements.push(id);
                 UI.showNotification(`Achievement: ${achievement.name}!`, 'achievement');
-                
+
+                // Audio feedback
+                Audio.playAchievement();
+
                 // Particle effect for achievement unlock
                 const countEl = document.querySelector('#achievements-tab h3');
                 if (countEl) {
diff --git a/js/audio.js b/js/audio.js
new file mode 100644
index 0000000..cfaeaf4
--- /dev/null
+++ b/js/audio.js
@@ -0,0 +1,119 @@
+/**
+ * Cosmic Garden - Audio System
+ * Web Audio API-based sound generation (no external assets needed)
+ */
+
+const Audio = {
+    ctx: null,
+    enabled: true,
+    
+    /**
+     * Initialize audio context (must be called after user interaction)
+     */
+    init() {
+        try {
+            window.AudioContext = window.AudioContext || window.webkitAudioContext;
+            this.ctx = new AudioContext();
+            console.log('🔊 Audio system initialized');
+        } catch (e) {
+            console.warn('Web Audio API not supported');
+            this.enabled = false;
+        }
+    },
+    
+    /**
+     * Resume audio context (for browsers that suspend it)
+     */
+    resume() {
+        if (this.ctx && this.ctx.state === 'suspended') {
+            this.ctx.resume();
+        }
+    },
+    
+    /**
+     * Create a simple tone
+     */
+    playTone(frequency, duration, type = 'sine', volume = 0.1) {
+        if (!this.enabled || !this.ctx) return;
+        
+        const osc = this.ctx.createOscillator();
+        const gain = this.ctx.createGain();
+        
+        osc.type = type;
+        osc.frequency.setValueAtTime(frequency, this.ctx.currentTime);
+        
+        gain.gain.setValueAtTime(volume, this.ctx.currentTime);
+        gain.gain.exponentialRampToValueAtTime(0.01, this.ctx.currentTime + duration);
+        
+        osc.connect(gain);
+        gain.connect(this.ctx.destination);
+        
+        osc.start(this.ctx.currentTime);
+        osc.stop(this.ctx.currentTime + duration);
+    },
+    
+    /**
+     * Play a rising arpeggio (for clicks)
+     */
+    playClick() {
+        if (!this.enabled) return;
+        this.resume();
+        
+        const now = this.ctx.currentTime;
+        const notes = [523.25, 659.25, 783.99]; // C major chord
+        
+        notes.forEach((freq, i) => {
+            setTimeout(() => {
+                this.playTone(freq, 0.1, 'sine', 0.08);
+            }, i * 30);
+        });
+    },
+    
+    /**
+     * Play a purchase sound (descending chime)
+     */
+    playPurchase() {
+        if (!this.enabled) return;
+        this.resume();
+        
+        const notes = [783.99, 659.25, 523.25]; // G - E - C
+        
+        notes.forEach((freq, i) => {
+            setTimeout(() => {
+                this.playTone(freq, 0.15, 'triangle', 0.1);
+            }, i * 50);
+        });
+    },
+    
+    /**
+     * Play an achievement unlock fanfare
+     */
+    playAchievement() {
+        if (!this.enabled) return;
+        this.resume();
+        
+        const notes = [523.25, 659.25, 783.99, 1046.50]; // C - E - G - C (high)
+        
+        notes.forEach((freq, i) => {
+            setTimeout(() => {
+                this.playTone(freq, 0.2, 'square', 0.06);
+            }, i * 80);
+        });
+    },
+    
+    /**
+     * Play a notification sound
+     */
+    playNotification() {
+        if (!this.enabled) return;
+        this.resume();
+        
+        this.playTone(880, 0.1, 'sine', 0.08);
+        setTimeout(() => {
+            this.playTone(1100, 0.15, 'sine', 0.08);
+        }, 100);
+    }
+};
+
+// Make globally accessible
+window.Audio = Audio;
diff --git a/js/game.js b/js/game.js
index d9fc4f5..e16b8f0 100644
--- a/js/game.js
+++ b/js/game.js
@@ -32,6 +32,7 @@ const Game = {
         this.setupEventListeners();
         this.startGameLoop();
         this.startAutoSave();
+        Audio.init();
         UI.init();
         Stats.init();
         console.log('🌌 Cosmic Garden initialized!');
@@ -98,7 +99,10 @@ const Game = {
         const amount = this.getClickPower();
         this.addStardust(amount);
         this.state.totalClicks++;
-        
+
+        // Audio feedback
+        Audio.playClick();
+
         // Visual feedback
         UI.showFloatingText(event.clientX, event.clientY, `+${this.formatNumber(amount)}`);
         Particles.createClickParticles(event.clientX, event.clientY, 8);
@@ -181,7 +185,10 @@ const Game = {
             this.state.buildings[buildingId] = (this.state.buildings[buildingId] || 0) + 1;
             
             UI.showNotification(`Bought ${BUILDINGS[buildingId].name}!`, 'success');
-            
+
+            // Audio feedback
+            Audio.playPurchase();
+
             // Particle effect on building purchase
             const card = document.getElementById(`building-${buildingId}`);
             if (card) {