diff --git a/micropython.cmake b/micropython.cmake
index d75f2de8..65bf0f6c 100644
--- a/micropython.cmake
+++ b/micropython.cmake
@@ -204,7 +204,6 @@ if(EXISTS ${MPAUDIO_MP3_SRC_DIR}/mp3dec.c)
     target_include_directories(usermod_mpaudio INTERFACE ${MPAUDIO_MP3_SRC_DIR})
     target_sources(usermod_mpaudio INTERFACE
         ${MPAUDIO_MP3_SRC_DIR}/bitstream.c
-        ${MPAUDIO_MP3_SRC_DIR}/buffers.c
         ${MPAUDIO_MP3_SRC_DIR}/dct32.c
         ${MPAUDIO_MP3_SRC_DIR}/dequant.c
         ${MPAUDIO_MP3_SRC_DIR}/dqchan.c
@@ -219,16 +218,18 @@ if(EXISTS ${MPAUDIO_MP3_SRC_DIR}/mp3dec.c)
         ${MPAUDIO_MP3_SRC_DIR}/subband.c
         ${MPAUDIO_MP3_SRC_DIR}/trigtabs.c
         ${MPAUDIO_SRC_DIR}/audiomp3/mp3_alloc.c
+        ${MPAUDIO_SRC_DIR}/audiomp3/mp3_buffers.c
         ${MPAUDIO_SRC_DIR}/audiomp3/MP3Decoder.c
         ${MPAUDIO_SRC_DIR}/audiomp3/module.c
     )
 
     # buffers.c's MPDEC_ALLOCATOR(x)/MPDEC_FREE(x) (used nowhere else in
-    # lib/mp3) route through this port's own mp3_alloc()/mp3_free() -- same
-    # -include + -fwrapv treatment as micropython.mk's buffers.o rule.
-    set_source_files_properties(${MPAUDIO_MP3_SRC_DIR}/buffers.c PROPERTIES
-        COMPILE_OPTIONS "-include;${MPAUDIO_SRC_DIR}/audiomp3/mp3_alloc.h;-DMPDEC_ALLOCATOR(x)=mp3_alloc(x);-DMPDEC_FREE(x)=mp3_free(x);-fwrapv"
-    )
+    # lib/mp3) route through this port's own mp3_alloc()/mp3_free(): the
+    # compile is src/audiomp3/mp3_buffers.c, which defines them and includes
+    # buffers.c. Not a set_source_files_properties() on buffers.c: that is
+    # scoped to this directory, and the ESP32 build compiles the linked copy of
+    # every usermod source from the project's directory, where it was lost
+    # (the buffers stayed static .bss, 23.8 kB of internal RAM).
 endif()
 
 # --- audiovorbis (VorbisDecoder) ---
diff --git a/src/audiomp3/mp3_buffers.c b/src/audiomp3/mp3_buffers.c
new file mode 100644
index 00000000..0b7c937a
--- /dev/null
+++ b/src/audiomp3/mp3_buffers.c
@@ -0,0 +1,26 @@
+// Helix's buffers.c, compiled with this port's allocator.
+//
+// buffers.c takes its eight decoder buffers from MPDEC_ALLOCATOR(x) when that
+// is defined, and from static arrays when it is not. The static arrays are
+// 23,816 bytes of .bss, which on an ESP32-S3 is internal RAM the Wi-Fi, BLE
+// and display drivers want. Defined, they come from the GC heap the first time
+// an MP3Decoder is made, as in CircuitPython's own port, and each decoder gets
+// buffers of its own.
+//
+// The macros used to be set with set_source_files_properties() in
+// micropython.cmake. That property belongs to the directory that sets it, and
+// the ESP32 build compiles every module source twice, once in the main
+// component's directory and once more in the project's: the linked copy was
+// the one without the flags. Defining the macros in a file of our own works
+// the same in every build, whichever directory compiles it. buffers.c itself
+// is not changed.
+//
+// SPDX-FileCopyrightText: Copyright (c) 2026 PyDevices
+// SPDX-License-Identifier: MIT
+
+#include "audiomp3/mp3_alloc.h"
+
+#define MPDEC_ALLOCATOR(x) mp3_alloc(x)
+#define MPDEC_FREE(x) mp3_free(x)
+
+#include "buffers.c"