fix: apply_sync ffmpeg streamcpy and filter

apply_sync.py

@@ -57,6 +57,23 @@ def probe_audio_streams(path: str) -> list[dict]:
         return []
 
 
+def _codec_args(codec: str, out_idx: int) -> list[str]:
+    """Return ffmpeg codec args for a given audio codec string."""
+    lossless = {"flac", "alac", "pcm_s16le", "pcm_s24le", "pcm_s32le", "pcm_f32le", "pcm_f64le"}
+    if codec in lossless:
+        return [f"-c:a:{out_idx}", codec]
+    # lossy codecs: use original where possible
+    codec_map = {
+        "aac": "aac", "mp3": "libmp3lame", "opus": "libopus",
+        "vorbis": "libvorbis", "ac3": "ac3", "eac3": "eac3",
+    }
+    enc = codec_map.get(codec, "aac")
+    args = [f"-c:a:{out_idx}", enc]
+    if enc != codec or codec not in lossless:
+        args += [f"-b:a:{out_idx}", "192k"]
+    return args
+
+
 def build_ffmpeg_cmd(file_a: str, file_b: str, lag_sec: float, out_path: str) -> list[str]:
     """Build ffmpeg command to mux A video + A audio + delayed B audio streams."""
     b_streams = probe_audio_streams(file_b)
@@ -67,6 +84,7 @@ def build_ffmpeg_cmd(file_a: str, file_b: str, lag_sec: float, out_path: str) ->
     set_german = (len(b_streams) == 1 and b_streams[0]["language"] in ("und", "unknown", ""))
 
     # Build filter_complex for B audio streams
+    # Negative adelay trims the start (B ahead of A), positive adds silence (B behind A)
     filters = []
     for i, s in enumerate(b_streams):
         filters.append(f"[1:a:{i}]adelay={lag_ms:.0f}|{lag_ms:.0f}:all=1[d{i}]")
@@ -81,11 +99,15 @@ def build_ffmpeg_cmd(file_a: str, file_b: str, lag_sec: float, out_path: str) ->
     for i in range(len(b_streams)):
         cmd += ["-map", f"[d{i}]"]
 
-    # Codecs: copy everything (lossless)
-    cmd += ["-c:v", "copy", "-c:a", "copy"]
+    # Codecs: copy video + A's audio, re-encode B's filtered audio with same codec
+    n_a_audio = len(probe_audio_streams(file_a))
+    cmd += ["-c:v", "copy"]
+    for i in range(n_a_audio):
+        cmd += [f"-c:a:{i}", "copy"]
+    for i, s in enumerate(b_streams):
+        cmd += _codec_args(s["codec"], n_a_audio + i)
 
     # Language metadata for B audio streams (they become the last N audio streams in output)
-    n_a_audio = len(probe_audio_streams(file_a))
     for i, s in enumerate(b_streams):
         out_audio_idx = n_a_audio + i  # 0-based index in output audio streams
         if set_german and i == 0: