diff --git a/.env.example b/.env.example
index e76874a..f24a9ec 100644
--- a/.env.example
+++ b/.env.example
@@ -39,7 +39,11 @@ MODELS_DIR=./models
 # ACESTEP_CPP_BRANCH=main
 
 # ── Storage ───────────────────────────────────────────────────────────────────
-AUDIO_DIR=./public/audio
+# Audio directory for generated songs and uploaded reference tracks.
+# Relative paths are resolved from the project root (APP_ROOT).
+# This is the single source of truth: LocalStorageProvider (writes),
+# Express /audio/ endpoint (serves), and the spawn service (reads) all use it.
+AUDIO_DIR=./server/public/audio
 
 # ── Auth ──────────────────────────────────────────────────────────────────────
 # Change this to a long random string in any multi-user or network-exposed setup.
diff --git a/backend/README.md b/backend/README.md
deleted file mode 100644
index cfd75ea..0000000
--- a/backend/README.md
+++ /dev/null
@@ -1,43 +0,0 @@
-# backend/ — design note
-
-## Why there is no custom C++ HTTP server here
-
-An earlier design wrapped `acestep-generate` in a second C++ HTTP server process.
-That was removed because it added unnecessary complexity:
-
-| Problem | Impact |
-|---------|--------|
-| Two processes to manage (Node.js + C++ server) | harder to deploy, restart, monitor |
-| C++ server used `popen()` with shell-built strings | fragile, platform-specific, injection surface |
-| LoRA state split across two processes | race conditions, stale cache |
-| Extra HTTP hop for every generation request | added latency and error surface |
-| Users need to build *two* C++ projects | poor DX |
-
-## Current architecture
-
-```
-Browser
-  │
-  │ HTTP
-  ▼
-Node.js Express (port 3001)
-  │  handles: auth, songs DB, playlists, audio storage, job queue
-  │
-  │ child_process.spawn(bin, args, { shell: false })
-  ▼
-acestep-generate  ←── GGUF model on GPU/CPU
-  │
-  └─► writes audio files → ./public/audio/
-```
-
-The Node.js server reads `ACESTEP_BIN` from `.env` and spawns `acestep-generate`
-directly — the same pattern used by llama.cpp, whisper.cpp, and similar tools.
-No shell is involved, so there is no injection risk.
-
-## When a separate HTTP server *would* make sense
-
-If `acestep.cpp` ever ships a **built-in** HTTP server mode (like `llama-server`),
-you can point `ACESTEP_API_URL` at it and leave `ACESTEP_BIN` empty.
-The Node.js service already has an HTTP-client fallback for exactly this case.
-
-See `server/src/services/acestep.ts` for the dual-mode implementation.
diff --git a/server/src/config/index.ts b/server/src/config/index.ts
index 14269e8..7abb41c 100644
--- a/server/src/config/index.ts
+++ b/server/src/config/index.ts
@@ -224,10 +224,11 @@ export const config = {
 
   storage: {
     provider: 'local' as const,
-    // Audio directory must match where LocalStorageProvider writes files and
-    // where Express serves /audio/ from (server/src/index.ts: '../public/audio').
-    // Both resolve to <server_root>/public/audio, so we use SERVER_ROOT here.
-    // AUDIO_DIR env override is still supported (resolved against APP_ROOT).
+    // Single source of truth for the audio directory.
+    // LocalStorageProvider, Express (/audio/), and the spawn service all read
+    // this value so they always point at the same filesystem location.
+    // Default: <server_root>/public/audio (SERVER_ROOT = server/).
+    // Override via AUDIO_DIR in .env (relative paths are resolved from APP_ROOT).
     audioDir: resolveFromRoot(process.env.AUDIO_DIR || path.join(SERVER_ROOT, 'public', 'audio')),
   },
 
diff --git a/server/src/index.ts b/server/src/index.ts
index f6bdbfb..e336f78 100644
--- a/server/src/index.ts
+++ b/server/src/index.ts
@@ -79,8 +79,9 @@ app.use(cors({
 
 app.use(express.json());
 
-// Serve static audio files
-app.use('/audio', express.static(path.join(__dirname, '../public/audio')));
+// Serve static audio files from the configured audio directory so that any
+// AUDIO_DIR env override is honoured consistently across upload, spawn, and serving.
+app.use('/audio', express.static(config.storage.audioDir));
 
 // Audio Editor (AudioMass) - needs relaxed CSP for inline scripts and external images
 app.use('/editor', (req, res, next) => {
diff --git a/server/src/services/acestep.ts b/server/src/services/acestep.ts
index 53663a9..1e40d8e 100644
--- a/server/src/services/acestep.ts
+++ b/server/src/services/acestep.ts
@@ -640,7 +640,8 @@ async function runViaSpawn(
     const batchSize = Math.min(Math.max(params.batchSize ?? 1, 1), 8);
     if (batchSize > 1) ditArgs.push('--batch', String(batchSize));
 
-    // Cover and repaint modes both require a source audio file
+    // Cover and repaint modes both require a source audio file.
+    // dit-vae reads WAV or MP3 natively (via dr_wav / dr_mp3 in audio.h).
     if (params.sourceAudioUrl) {
       const srcAudioPath = resolveAudioPath(params.sourceAudioUrl);
       ditArgs.push('--src-audio', srcAudioPath);
diff --git a/server/src/services/storage/local.ts b/server/src/services/storage/local.ts
index 903582c..94f5979 100644
--- a/server/src/services/storage/local.ts
+++ b/server/src/services/storage/local.ts
@@ -1,17 +1,16 @@
 import { writeFile, unlink, stat, mkdir, copyFile } from 'fs/promises';
 import path from 'path';
-import { fileURLToPath } from 'url';
 import type { StorageProvider } from './index.js';
-
-const __filename = fileURLToPath(import.meta.url);
-const __dirname = path.dirname(__filename);
-const AUDIO_DIR = path.join(__dirname, '../../../public/audio');
+import { config } from '../../config/index.js';
 
 export class LocalStorageProvider implements StorageProvider {
   private audioDir: string;
 
   constructor() {
-    this.audioDir = AUDIO_DIR;
+    // Derive the audio directory from the central config so that the storage
+    // provider always writes to the same location the spawn service resolves
+    // paths from (config.storage.audioDir, which honours the AUDIO_DIR env var).
+    this.audioDir = config.storage.audioDir;
   }
 
   async upload(key: string, data: Buffer, _contentType: string): Promise<string> {