diff --git a/audio/out/ao.c b/audio/out/ao.c
index 73131020c0..fa6cc9e76d 100644
--- a/audio/out/ao.c
+++ b/audio/out/ao.c
@@ -444,10 +444,14 @@ int ao_query_and_reset_events(struct ao *ao, int events)
     return atomic_fetch_and(&ao->events_, ~(unsigned)events) & events;
 }
 
-void ao_add_events(struct ao *ao, int events)
+// Returns events that were set by this calls.
+int ao_add_events(struct ao *ao, int events)
 {
-    atomic_fetch_or(&ao->events_, events);
-    ao->wakeup_cb(ao->wakeup_ctx);
+    unsigned prev_events = atomic_fetch_or(&ao->events_, events);
+    unsigned new = events & ~prev_events;
+    if (new)
+        ao->wakeup_cb(ao->wakeup_ctx);
+    return new;
 }
 
 // Request that the player core destroys and recreates the AO. Fully thread-safe.
@@ -462,12 +466,13 @@ void ao_hotplug_event(struct ao *ao)
     ao_add_events(ao, AO_EVENT_HOTPLUG);
 }
 
-void ao_underrun_event(struct ao *ao)
+// Returns whether this call actually set a new underrun flag.
+bool ao_underrun_event(struct ao *ao)
 {
-    // Racy check, but it's just for the message.
-    if (!(atomic_load(&ao->events_) & AO_EVENT_UNDERRUN))
+    bool new_underrun = ao_add_events(ao, AO_EVENT_UNDERRUN);
+    if (new_underrun)
         MP_WARN(ao, "Device underrun detected.\n");
-    ao_add_events(ao, AO_EVENT_UNDERRUN);
+    return new_underrun;
 }
 
 bool ao_chmap_sel_adjust(struct ao *ao, const struct mp_chmap_sel *s,
diff --git a/audio/out/ao.h b/audio/out/ao.h
index f6b984ff30..eb9e29dacb 100644
--- a/audio/out/ao.h
+++ b/audio/out/ao.h
@@ -110,7 +110,7 @@ void ao_resume(struct ao *ao);
 void ao_drain(struct ao *ao);
 bool ao_eof_reached(struct ao *ao);
 int ao_query_and_reset_events(struct ao *ao, int events);
-void ao_add_events(struct ao *ao, int events);
+int ao_add_events(struct ao *ao, int events);
 void ao_unblock(struct ao *ao);
 void ao_request_reload(struct ao *ao);
 void ao_hotplug_event(struct ao *ao);
diff --git a/audio/out/internal.h b/audio/out/internal.h
index d1d7482ec1..7d1d744991 100644
--- a/audio/out/internal.h
+++ b/audio/out/internal.h
@@ -211,7 +211,7 @@ struct pollfd;
 int ao_wait_poll(struct ao *ao, struct pollfd *fds, int num_fds,
                  pthread_mutex_t *lock);
 void ao_wakeup_poll(struct ao *ao);
-void ao_underrun_event(struct ao *ao);
+bool ao_underrun_event(struct ao *ao);
 
 bool ao_chmap_sel_adjust(struct ao *ao, const struct mp_chmap_sel *s,
                          struct mp_chmap *map);