diff --git a/packages/engine/src/services/audioMixer.order.test.ts b/packages/engine/src/services/audioMixer.order.test.ts
new file mode 100644
index 0000000000..88be132bbb
--- /dev/null
+++ b/packages/engine/src/services/audioMixer.order.test.ts
@@ -0,0 +1,95 @@
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { basename, join } from "node:path";
+import { setImmediate } from "node:timers/promises";
+import { afterEach, expect, it, vi } from "vitest";
+
+const { runFfmpegMock } = vi.hoisted(() => ({ runFfmpegMock: vi.fn() }));
+vi.mock("../utils/runFfmpeg.js", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("../utils/runFfmpeg.js")>()),
+  runFfmpeg: runFfmpegMock,
+}));
+vi.mock("../utils/ffprobe.js", () => ({
+  extractAudioMetadata: vi.fn(async () => ({ durationSeconds: 1, channels: 2 })),
+}));
+
+import { processCompositionAudio, type AudioElement } from "./audioMixer.js";
+
+const dirs: string[] = [];
+afterEach(() => {
+  runFfmpegMock.mockReset();
+  for (const dir of dirs.splice(0)) rmSync(dir, { recursive: true, force: true });
+});
+
+it.each(["flat", "one bus", "two buses"])(
+  "keeps authored mix input order when preparations finish in reverse (%s)",
+  async (layout) => {
+    const dir = mkdtempSync(join(tmpdir(), "hf-audio-order-"));
+    dirs.push(dir);
+    const ids =
+      layout === "two buses"
+        ? ["first", "second", "third", "fourth"]
+        : ["first", "second", "third"];
+    const elements: AudioElement[] = ids.map((id, index) => {
+      writeFileSync(join(dir, `${id}.wav`), "RIFF0000WAVE");
+      return {
+        id,
+        src: `${id}.wav`,
+        type: "audio",
+        start: 0,
+        end: 1,
+        mediaStart: 0,
+        layer: 0,
+        volume: 1,
+        ...(layout === "flat"
+          ? {}
+          : { groupId: layout === "one bus" ? "bus" : `bus-${index % 2}`, groupVolume: 1 }),
+      };
+    });
+    const pending = new Map<string, () => void>();
+    const completed: string[] = [];
+    const mixes: string[][] = [];
+    let allStarted = () => {};
+    const started = new Promise<void>((resolve) => {
+      allStarted = resolve;
+    });
+    runFfmpegMock.mockImplementation(async (args: string[]) => {
+      const out = args.at(-1)!;
+      if (out.endsWith("-trimmed.wav")) {
+        await new Promise<void>((resolve) => {
+          pending.set(basename(out), resolve);
+          if (pending.size === elements.length) allStarted();
+        });
+        completed.push(basename(out));
+      } else {
+        mixes.push(args.flatMap((arg, i) => (arg === "-i" ? [basename(args[i + 1]!)] : [])));
+      }
+      writeFileSync(out, "RIFF0000WAVE");
+      return { success: true, durationMs: 1, stderr: "", exitCode: 0 };
+    });
+    const result = processCompositionAudio(
+      elements,
+      dir,
+      join(dir, "work"),
+      join(dir, "out.m4a"),
+      1,
+    );
+    await started;
+    const reversed = [...ids].reverse();
+    for (const id of reversed) {
+      pending.get(`${id}-trimmed.wav`)!();
+      await setImmediate();
+    }
+    expect((await result).success).toBe(true);
+    expect(completed).toEqual(reversed.map((id) => `${id}-trimmed.wav`));
+    if (layout === "two buses") {
+      expect(mixes).toEqual([
+        ["first-trimmed.wav", "third-trimmed.wav"],
+        ["second-trimmed.wav", "fourth-trimmed.wav"],
+        ["group-bus-0-0.wav", "group-bus-1-1.wav"],
+      ]);
+    } else {
+      expect(mixes[0]).toEqual(["first-trimmed.wav", "second-trimmed.wav", "third-trimmed.wav"]);
+    }
+  },
+);
diff --git a/packages/engine/src/services/audioMixer.ts b/packages/engine/src/services/audioMixer.ts
index 2d6ebb9263..42c0b3cc82 100644
--- a/packages/engine/src/services/audioMixer.ts
+++ b/packages/engine/src/services/audioMixer.ts
@@ -1123,6 +1123,7 @@ export async function processCompositionAudio(
 ): Promise<MixResult> {
   const startMs = Date.now();
   const tracks: AudioTrack[] = [];
+  const preparedTracks = new Array<AudioTrack | undefined>(elements.length);
   const failures: AudioProcessingFailure[] = [];
   // Grouped members are diverted here instead of `tracks` during element
   // processing, then summed into one processed track per group before the
@@ -1154,7 +1155,7 @@ export async function processCompositionAudio(
   }
 
   await Promise.all(
-    elements.map(async (element) => {
+    elements.map(async (element, index) => {
       if (effectiveSignal.aborted) {
         failures.push({
           stage: "cancelled",
@@ -1386,24 +1387,7 @@ export async function processCompositionAudio(
           ...(tailSeconds > 0 ? { tailSeconds } : {}),
         };
 
-        // A grouped member keeps every bit of its OWN processing above
-        // (FX, envelope) exactly like an ungrouped track — it just lands in
-        // the group's bucket instead of the flat list, to be summed into one
-        // processed bus below rather than mixed directly.
-        if (element.groupId) {
-          const bucket = groupTracks.get(element.groupId);
-          if (bucket) bucket.push(track);
-          else groupTracks.set(element.groupId, [track]);
-          if (!groupMeta.has(element.groupId)) {
-            groupMeta.set(element.groupId, {
-              ...(element.groupFxChain ? { fxChain: element.groupFxChain } : {}),
-              ...(element.groupAutomation ? { automation: element.groupAutomation } : {}),
-              volume: element.groupVolume ?? 1,
-            });
-          }
-        } else {
-          tracks.push(track);
-        }
+        preparedTracks[index] = track;
       } catch (err: unknown) {
         // An FX failure is fatal for the whole mix. Every other failure mode
         // degrades gracefully — the track drops and siblings continue — but
@@ -1436,6 +1420,27 @@ export async function processCompositionAudio(
     throw err;
   });
 
+  // FFmpeg's floating-point sum depends on input order. Keep parallel prep,
+  // but assemble flat tracks, group members and groups in authored order.
+  elements.forEach((element, index) => {
+    const track = preparedTracks[index];
+    if (!track) return;
+    if (element.groupId) {
+      const bucket = groupTracks.get(element.groupId);
+      if (bucket) bucket.push(track);
+      else groupTracks.set(element.groupId, [track]);
+      if (!groupMeta.has(element.groupId)) {
+        groupMeta.set(element.groupId, {
+          ...(element.groupFxChain ? { fxChain: element.groupFxChain } : {}),
+          ...(element.groupAutomation ? { automation: element.groupAutomation } : {}),
+          volume: element.groupVolume ?? 1,
+        });
+      }
+    } else {
+      tracks.push(track);
+    }
+  });
+
   // Never turn a per-track preparation failure into a successful partial mix.
   // The producer only surfaces audio failures when `success` is false; mixing
   // the remaining tracks made the omitted cue indistinguishable from a valid