diff --git a/src/lib/ffmpeg.ts b/src/lib/ffmpeg.ts
index f28aaa0e..707d2955 100644
--- a/src/lib/ffmpeg.ts
+++ b/src/lib/ffmpeg.ts
@@ -91,6 +91,7 @@ export function terminateFFmpeg() {
   ffmpegInstance = null;
 }
 
+/** Generates a unique session ID used to isolate FFmpeg file names across concurrent exports. */
 function buildSessionId(): string {
   if (typeof crypto !== "undefined" && "randomUUID" in crypto) {
     return crypto.randomUUID();
@@ -195,6 +196,7 @@ function buildAudioTrimFilter(recipe: EditRecipe): string {
   return `atrim=start=${recipe.trimStart}:end=${end},asetpts=PTS-STARTPTS`;
 }
 
+type OutputFormat = "mp4" | "webm" | "mkv";
 function buildArguments(
   recipe: EditRecipe,
   format: "mp4" | "webm" | "mkv" | "gif",
@@ -242,27 +244,27 @@ function buildArguments(
       videoOut = "[vbase]";
     }
 
-    if (hasOverlay) {
-      const scaledW = overlayOptions!.size;
-      const alpha = (overlayOptions!.opacity / 100).toFixed(2);
+    if (hasOverlay && overlayOptions) {
+      const scaledW = overlayOptions.size ?? 100;
+      const alpha = ((overlayOptions.opacity ?? 100) / 100).toFixed(2);
       const posMap: Record<string, string> = {
         "top-left":     "20:20",
         "top-right":    "W-w-20:20",
         "bottom-left":  "20:H-h-20",
         "bottom-right": "W-w-20:H-h-20",
       };
-      const pos = posMap[overlayOptions!.position] ?? "W-w-20:H-h-20";
+      const pos = overlayOptions.position ? posMap[overlayOptions.position] : "W-w-20:H-h-20";
+      const finalPos = pos ?? "W-w-20:H-h-20";
       filterParts.push(`[${overlayIdx}:v]scale=${scaledW}:-2,format=rgba,colorchannelmixer=aa=${alpha}[logo]`);
-      filterParts.push(`${videoOut}[logo]overlay=${pos}[vout]`);
+      filterParts.push(`${videoOut}[logo]overlay=${finalPos}[vout]`);
       videoOut = "[vout]";
     }
 
     let audioOut = "";
-    if (shouldKeepAudio) {
-      if (hasMusicTrack) {
-        const musicVol = (musicOptions!.musicVolume / 100).toFixed(2);
+          if (hasMusicTrack && musicOptions) {
+        const musicVol = ((musicOptions.musicVolume ?? 100) / 100).toFixed(2);
         if (hasOriginalAudio) {
-          const origVol  = (musicOptions!.originalAudioVolume / 100).toFixed(2);
+          const origVol = ((musicOptions.originalAudioVolume ?? 100) / 100).toFixed(2);
           const origChain = afParts.length > 0
             ? `[0:a]${afParts.join(",")},volume=${origVol}[orig]`
             : `[0:a]volume=${origVol}[orig]`;
@@ -274,10 +276,7 @@ function buildArguments(
           filterParts.push(`[${musicIdx}:a]volume=${musicVol}[aout]`);
           audioOut = "[aout]";
         }
-      } else if (hasOriginalAudio && af) {
-        filterParts.push(`[0:a]${af}[aout]`);
-        audioOut = "[aout]";
-      }
+      
     }
 
     if (filterParts.length > 0) {
@@ -369,7 +368,12 @@ export async function exportVideo(
     }
   };
 
-  const { filename: outputName, mimeType } = getOutputConfig(recipe.format);
+  // Format handling - single source of truth
+  const outputFormat: OutputFormat = 
+    recipe.format === "webm" ? "webm" :
+    recipe.format === "mkv" ? "mkv" : "mp4";
+
+  const { filename: outputName, mimeType } = getOutputConfig(outputFormat);
   const fallbackOutputName = `fallback_${sessionId}.webm`;
   const paletteName = `palette_${sessionId}.png`;
   const cleanupFiles = new Set<string>([inputName, outputName, fallbackOutputName, paletteName]);
@@ -539,7 +543,7 @@ export async function exportVideo(
       size: blob.size,
       width: targetW,
       height: targetH,
-      format: recipe.format as "mp4" | "webm" | "mkv",
+      format: outputFormat,
     };
   } finally {
     ffmpeg.off("progress", handleProgress);
@@ -551,6 +555,7 @@ export async function exportVideo(
   }
 }
 
+/** Formats a byte count as a human-readable string (KB or MB). */
 export function formatBytes(bytes: number): string {
   if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
   return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
diff --git a/src/lib/types.ts b/src/lib/types.ts
index 30cab3aa..3c61c58c 100644
--- a/src/lib/types.ts
+++ b/src/lib/types.ts
@@ -42,18 +42,19 @@ export type OverlayPosition =
   | "bottom-left"
   | "bottom-right";
 
+
 export interface ImageOverlayOptions {
-  file: File | null;
-  position: OverlayPosition;
-  size: number;
-  opacity: number;
+  file?: File | null;
+  position?: OverlayPosition;
+  size?: number;
+  opacity?: number;
 }
 
 export interface BackgroundMusicOptions {
-  file: File | null;
-  musicVolume: number;
-  originalAudioVolume: number;
-  loopMusic: boolean;
+  file?: File | null;
+  musicVolume?: number;
+  originalAudioVolume?: number;
+  loopMusic?: boolean;
 }
 
 export interface ExportResult {