fix: transcode audio to MP3 instead of stream-copy in ffmpeg

[sweetmantech]

Summary

• ffmpeg -c copy fails when input is WAV with .mp3 extension (common with Slack uploads)
• Replace with -c:a libmp3lame -q:a 2 to transcode any audio format to MP3

Context

User-attached audio from Slack can be WAV files. The pipeline saves them as .mp3 but ffmpeg detects the actual codec (pcm_s16le) and refuses to stream-copy into an MP3 container.

Test plan

• 209/209 tests pass
• E2E: Upload WAV file via Slack → verify video renders successfully

🤖 Generated with Claude Code

Summary by CodeRabbit

Bug Fixes

• Audio clipping has been updated to re-encode segments with improved quality settings during upload, replacing the previous stream copying method for better consistency.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: aca0229f-b80a-46c6-88e4-b4c5d4bd8e48

📥 Commits

Reviewing files that changed from the base of the PR and between b32f8ca and 6b9824c.

📒 Files selected for processing (1)

• src/tasks/createContentTask.ts

📝 Walkthrough

Walkthrough

Modified FFmpeg command in audio clipping functionality to re-encode audio segments as MP3 with libmp3lame encoder at quality level 2, replacing the previous stream copy approach. The overall function flow and interface remain unchanged.

Changes

Cohort / File(s)	Summary
Audio Processing Configuration src/content/generateAudioVideo.ts	Updated FFmpeg audio clipping parameters from -c copy (no re-encoding) to -c:a libmp3lame -q:a 2 (MP3 re-encoding with quality level 2).

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 Hops with glee, the audio flows,
From copy-paste to libmp3 prose,
Quality level two, a cleaner sound,
Where MP3 magic now is found! 🎵

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: switching from stream-copy to MP3 transcoding in ffmpeg, which is the primary fix for handling WAV files misnamed as MP3.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/ffmpeg-wav-to-mp3-transcode

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[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

src/content/generateAudioVideo.ts (1)

106-114: ⚠️ Potential issue | 🔴 Critical

Ensure libmp3lame encoder is available in the FFmpeg build before deployment.

The code change now explicitly requires the libmp3lame encoder (-c:a libmp3lame), but it is not available in the current runtime environment. The clipping function will fail at runtime with an "Unknown encoder" error. Either add libmp3lame to the FFmpeg build, use a different encoder that is available, or re-evaluate the transcoding approach.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@src/content/generateAudioVideo.ts` around lines 106 - 114, The ffmpeg
invocation in generateAudioVideo.ts explicitly requests the libmp3lame encoder
in the execFileAsync call (args include "-c:a", "libmp3lame"), which will fail
if that encoder isn't present; either update the runtime FFmpeg build to include
libmp3lame or change the encoder argument to one available in your environment
(e.g., "aac" or "libopus") and ensure clippedPath/extension and any downstream
consumers match the chosen codec; update the execFileAsync call, related
variable handling (inputPath, clippedPath) and deployment/docs to reflect the
chosen encoder so runtime failures are avoided.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Outside diff comments:
In `@src/content/generateAudioVideo.ts`:
- Around line 106-114: The ffmpeg invocation in generateAudioVideo.ts explicitly
requests the libmp3lame encoder in the execFileAsync call (args include "-c:a",
"libmp3lame"), which will fail if that encoder isn't present; either update the
runtime FFmpeg build to include libmp3lame or change the encoder argument to one
available in your environment (e.g., "aac" or "libopus") and ensure
clippedPath/extension and any downstream consumers match the chosen codec;
update the execFileAsync call, related variable handling (inputPath,
clippedPath) and deployment/docs to reflect the chosen encoder so runtime
failures are avoided.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9d62c363-55b9-4b43-9c47-d6057a3ab47a

📥 Commits

Reviewing files that changed from the base of the PR and between d974928 and b32f8ca.

📒 Files selected for processing (1)

• src/content/generateAudioVideo.ts