X11 audible bell is disabled by muffin, but nothing replaces it when audible-bell is false

Distribution

Debian 12 (bookworm); also reproducible on current Debian testing/unstable

Package version

Cinnamon 5.6.8 / muffin 5.6.4 (the code is unchanged in 6.6.x and in current master)

Graphics hardware in use

Not relevant (X11 session, any hardware)

Frequency

Always

Bug description

On X11 muffin turns off the X server's audible bell at startup, in
src/x11/meta-x11-display.c:

/* We are playing sounds using libcanberra support, we handle the
 * bell whether its an audible bell or a visible bell */
XkbChangeEnabledControls (x11_display->xdisplay,
                          XkbUseCoreKbd,
                          XkbAudibleBellMask,
                          0);

But muffin only plays a bell of its own when
org.cinnamon.desktop.wm.preferences audible-bell is true, and that key
defaults to false. With the default settings the bell is therefore simply
gone: the X/hardware beep is disabled by the window manager, and nothing is
played in its place.

This is a change from Cinnamon 4.8, where meta_bell_init() only called
XkbSelectEvents(..., XkbBellNotifyMask, ...) and left the X audible bell
enabled, so the beep worked out of the box.

There is a fallback that looks like it should cover exactly this case, in
src/x11/events.c:

if (!meta_bell_notify (display, window) &&
    meta_prefs_bell_is_audible ())
  {
    /* Force a classic bell if the libcanberra bell failed. */
    XkbForceDeviceBell (...);
  }

but it can never run: bell_audible_notify() in src/core/bell.c returns TRUE
unconditionally, without checking whether the sound was actually played, and it
is only reached when audible-bell is already true.

Steps to reproduce

1. Log into a Cinnamon X11 session with default settings (audible-bell false).
2. Open a terminal and run echo -e '\a'.
3. Nothing is heard — no sound file, and no PC speaker beep either (the pcspkr
   module being loaded makes no difference).
4. Setting audible-bell to true (System Settings > Accessibility > "Enable
   audio alerts") makes muffin play its own bell sound, confirming where the
   behaviour comes from.

Expected behavior

Either the X audible bell is left enabled when muffin is not going to play a
bell itself, or bell_audible_notify() reports the real outcome so that the
existing XkbForceDeviceBell() fallback can do its job.

Additional information

Reported in Debian as https://bugs.debian.org/1036121, after an upgrade from
Cinnamon 4.8 to 5.6.

Trace the bell flow through src/x11/meta-x11-display.c, src/x11/events.c, and src/core/bell.c, starting with the X11 startup control change and the terminal reproduction command. Compare the default audible-bell=false path with the enabled path and verify the existing fallback's return-value assumptions. Done means the default X11 session produces an audible bell without breaking the configured audio-alert behavior.