diff --git a/audio/masters/.gdignore b/audio/masters/.gdignore
new file mode 100644
index 0000000..e69de29
diff --git a/audio/masters/README.md b/audio/masters/README.md
new file mode 100644
index 0000000..169a487
--- /dev/null
+++ b/audio/masters/README.md
@@ -0,0 +1,18 @@
+# Audio masters
+
+The source files the shipped audio is made from. Versioned, but **not part of
+the game**: the empty `.gdignore` beside this file tells Godot to skip this
+folder entirely, so nothing here is imported into `res://` and nothing here
+is carried into an export.
+
+That split is the point. `SPACE_4.wav` is 59MB of 16-bit stereo; the theme
+the game actually loads is `../space_4.mp3`, the same 5m36s at LAME 160k in
+6.7MB. Shipping the WAV as well would put 59MB into every Android and web
+build for a file nothing opens.
+
+| master | shipped as | made with |
+| --- | --- | --- |
+| `SPACE_4.wav` | `../space_4.mp3` | `ffmpeg -i SPACE_4.wav -c:a libmp3lame -b:a 160k ../space_4.mp3` |
+
+The sound effects have no master here - they are generated outright by
+`../make_sfx.py`, which is their source.
diff --git a/audio/masters/SPACE_4.wav b/audio/masters/SPACE_4.wav
new file mode 100644
index 0000000..f24d23e
Binary files /dev/null and b/audio/masters/SPACE_4.wav differ
diff --git a/tests/run_tests.sh b/tests/run_tests.sh
index 6d61a56..6a25759 100644
--- a/tests/run_tests.sh
+++ b/tests/run_tests.sh
@@ -173,9 +173,12 @@ if [[ "$IN_PLACE" != "1" ]]; then
 	# keep being run.
 	find "$run_dir" -mindepth 1 -maxdepth 1 ! -name '.godot' -exec rm -rf {} +
 	# android/ and builds/ are export artefacts no test touches, and the .apk
-	# is 30MB of nothing useful here.
+	# is 30MB of nothing useful here. audio/masters/ is the same argument:
+	# Godot skips it (.gdignore) so no test can reach it, and it is 59MB of
+	# source WAV that would be copied on every single run.
 	tar -C "$PROJECT_DIR" \
 		--exclude=./.godot --exclude=./android --exclude=./builds --exclude='*.apk' \
+		--exclude=./audio/masters \
 		-cf - . 2>/dev/null | tar -C "$run_dir" -xf -
 fi