Document H3 audio VAE path

PLAN.md

@@ -99,3 +99,34 @@ Backends/experiments to evaluate later:
 
 - Attention backend sweeps after VAE quality is fixed: `sage2`, `sage3`, `sdpa`, plus future FlashAttention-4/Sol-Attn/KJ candidates.
 - Spectrum MiniMax H3 repo to inspect later: https://github.com/xmarre/ComfyUI-Spectrum-MiniMax-H3
+
+## H3 Audio VAE Notes
+
+Comfy source references:
+
+- `comfy/ldm/minimax/audio_vae.py`: `MiniMaxH3AudioVAE`
+- `comfy_extras/nodes_minimax_h3.py`: AV latent creation and audio guide encoding
+- `comfy_extras/nodes_lt_audio.py`: `LTXVAudioVAEDecode`, which also handles nested AV latents and calls `audio_vae.decode(audio_latent)`
+- `comfy_extras/nodes_audio.py`: generic audio save/preview and normalization helpers
+
+Local checkpoint:
+
+- Spark: `/home/daniel/aeon-spark-test/h3/comfy-models/vae/minimax_h3_audio_vae_fp32.safetensors`
+- Runtime mount: `/vae/minimax_h3_audio_vae_fp32.safetensors`
+- Shape inspection: 917 keys; top-level names include `dec_in_proj.*`, `decoder.*`, `encoder.*`, `latents_mean`, `latents_std`, `mean_proj.*`, `logs_proj.*`, and `pre_block.*`.
+
+Architecture summary:
+
+- This is not similar to the video ViT3D VAE. It is a DAC-lineage waveform encoder plus BigVGAN decoder.
+- Latents are normalized `[B, 32, 2, T]`: 32 channels, 2 stereo channels, 40 latent frames/sec.
+- Decoder output is stereo waveform `[B, 2, L]` at 32 kHz.
+- One audio latent frame equals 800 samples (`32000 / 40`).
+- Comfy decode path for nested H3 AV latent is effectively: select nested audio tensor, call `audio_vae.decode(audio_latent)`, then return waveform with sample rate 32000. VideoHelperSuite/Comfy audio save nodes use ffmpeg or audio helpers to save/mux.
+
+Direct runtime implementation path:
+
+- Change `sample_video_res_multistep()` to return both final video and final audio latents, or add a `sample_av_res_multistep()` wrapper that preserves the existing video-only API.
+- Port `MiniMaxH3AudioVAE` from Comfy into a standalone `audio_vae_decoder.py`, starting decoder-only if we only need generated audio.
+- Load `/vae/minimax_h3_audio_vae_fp32.safetensors` strictly in FP32, mirroring the video VAE precision lesson.
+- Add a latent-only decode tool that accepts saved audio latent `[1,32,2,T]`, writes WAV/FLAC at 32 kHz, and optionally muxes with MP4 using ffmpeg.
+- Validate first by capturing or decoding the same final audio latent through Comfy and direct, comparing waveform tensors before muxing.