diff --git a/internal/engine/transcoder.go b/internal/engine/transcoder.go
index 1752d6d..215f5bd 100644
--- a/internal/engine/transcoder.go
+++ b/internal/engine/transcoder.go
@@ -259,18 +259,34 @@ func buildFFmpegArgs(filePath string, opts TranscodeOpts) []string {
 			filterChain = "format=yuv420p,setparams=colorspace=bt709:color_trc=bt709:color_primaries=bt709:range=tv"
 		}
 		args = append(args, "-vf", filterChain)
-		args = append(args, "-c:a", "aac", "-b:a", coalesce(opts.AudioBitrate, "192k"))
+		// Force AAC-LC stereo 48 kHz so MSE's CHUNK_DEMUXER accepts the moov.
+		// 5.1 / 7.1 source streams produce a moov shape that MSE refuses to
+		// parse (the <video src=blob:> demuxer is more forgiving), so we
+		// always downmix to stereo and resample to 48 kHz here. Source
+		// material that's already stereo passes through losslessly aside
+		// from the re-encode.
+		args = append(args,
+			"-c:a", "aac",
+			"-b:a", coalesce(opts.AudioBitrate, "192k"),
+			"-ar", "48000",
+			"-ac", "2",
+		)
 	}
 
 	// Common output flags — fragmented MP4 to a single pipe.
-	// NO faststart: that flag rewrites the moov atom to the front of the
-	// file as a SECOND pass after encoding finishes, which means the
-	// browser never sees a moov until ffmpeg exits. For live transcoding
-	// we need empty_moov (write a placeholder up front) so MSE can start
-	// decoding the very first fragment. faststart is only safe for
-	// already-finished files.
+	//
+	//   * empty_moov + default_base_moof: write a header-only init segment
+	//     up front so MSE can start decoding before the file is finished.
+	//   * frag_duration=1s: cap each moof+mdat at ~1 second of media. Without
+	//     this, ffmpeg only splits at keyframes, which on a high-bitrate
+	//     1080p stream produces 8 MiB+ mdat boxes — MSE refuses to parse
+	//     the first fragment until the whole mdat lands, so playback never
+	//     starts.
+	//   * negative_cts_offsets: lets b-frames carry the right pts/dts so
+	//     decoders don't reset the playhead to 0 every fragment.
 	args = append(args,
-		"-movflags", "frag_keyframe+empty_moov+default_base_moof",
+		"-movflags", "+frag_keyframe+empty_moov+default_base_moof+negative_cts_offsets",
+		"-frag_duration", "1000000",
 		"-f", "mp4",
 		"pipe:1",
 	)