Extracting WEM Audio (Wwise / Helldivers 2 mods)

How to pull playable audio out of Wwise game-audio files — useful for sourcing sound assets (engine loops, weapon SFX) to ship in a 7DTD mod. Verified against Helldivers 2 sound-mod files (June 2026).

What the files are

Helldivers 2 audio mods ship as Autodesk Stingray archives:

• <hash>.patch_0 — an archive containing a Wwise SoundBank (BKHD magic) plus embedded WEM audio (each a RIFF....WAVE block).
• <hash>.patch_0.stream — a sibling container holding the full-quality streamed WEMs as their own complete RIFF/WAVE files. The .patch_0 only carries a short ~1s prefetch copy of each streamed sound; the real audio is in .stream.
• manifest.json — mod metadata (option names, descriptions, icons).

WEM codec is Wwise Vorbis (fmt chunk format code 0xFFFF). ffmpeg cannot decode this — you need vgmstream (or ww2ogg + revorb).

Tooling

• vgmstream-cli — the one decoder that handles Wwise Vorbis WEM directly. Download the Windows build: https://github.com/vgmstream/vgmstream/releases/latest/download/vgmstream-win64.zip Decode: vgmstream-cli.exe -o out.wav in.wem
• ffmpeg/ffprobe — only for inspecting/converting the resulting WAVs.

Gotcha: vgmstream-cli is a Windows exe — give it real Windows paths. A cygwin/Git-Bash /tmp/... path makes it print failed opening <file>.

Extraction recipe

1. Carve every WEM from both .patch_0 and .patch_0.stream by scanning for RIFF, reading the little-endian size at offset+4, and slicing data[i : i+4+size] (skip any slice that runs past EOF — those are truncated prefetch entries whose data lives in .stream).
2. Decode each carved .wem to .wav with vgmstream-cli; treat a returncode==0 + >1KB WAV as success.
3. The .stream WEMs are self-contained complete RIFFs — no header stitching needed. (Stitching the bank's prefetch header onto raw .stream bytes does NOT work — vgmstream rejects it. Just carve the .stream directly.)
4. Deduplicate by content hash: a single bank references the same sound many times, so 386 raw WEMs collapsed to ~339 unique WAVs.

The full-quality loops (e.g. a 19.35s engine loop) come from the .stream container; the in-bank copies of those same sounds are only ~1.3s prefetch.

Don't commit the output

Extracted WAVs are large (278 MB for one mod) and vgmstream is a third-party binary — gitignore both rather than committing. Also: extracted content is someone else's mod asset; fine for personal reference, mind licensing before shipping any of it.