diff --git a/alsa.c b/alsa.c
index ebfaeab..48de434 100644
--- a/alsa.c
+++ b/alsa.c
@@ -25,10 +25,23 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <limits.h>
+#include <pthread.h>
 #define ALSA_PCM_NEW_HW_PARAMS_API
 #include <alsa/asoundlib.h>
 #include "espeakup.h"
 
+static pthread_mutex_t audio_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+void lock_audio_mutex(void)
+{
+	pthread_mutex_lock(&audio_mutex);
+}
+
+void unlock_audio_mutex(void)
+{
+	pthread_mutex_unlock(&audio_mutex);
+}
+
 int minimum(int x, int y)
 {
 	if (x <= y)
@@ -57,11 +70,15 @@ static int alsa_play_callback(short *audio, int numsamples,
 		snd_pcm_prepare(handle);
 
 	while (numsamples > 0) {
+		lock_audio_mutex();
 		if (stopped) {
 			snd_pcm_drop(handle);
 			stopped = 0;
+			unlock_audio_mutex();
 			return 1;
 		}
+		unlock_audio_mutex();
+
 		avail = snd_pcm_avail_update(handle);
 		if (avail <= 0)
 			continue;
diff --git a/espeak_sound.c b/espeak_sound.c
index f89538a..c9dcd82 100644
--- a/espeak_sound.c
+++ b/espeak_sound.c
@@ -5,3 +5,18 @@ int init_audio(void)
 	audio_callback = NULL;
 	return 0;
 }
+
+/*
+ * lock_audio_mutex and unlock_audio_mutex are no-ops if we use native
+ * sound support.  The stopped variable is never read; no need to protect it.
+ */
+
+void lock_audio_mutex(void)
+{
+	return;
+}
+
+void unlock_audio_mutex(void)
+{
+	return;
+}
diff --git a/espeakup.h b/espeakup.h
index 5a31cdf..a289e32 100644
--- a/espeakup.h
+++ b/espeakup.h
@@ -79,6 +79,8 @@ extern void close_softsynth(void);
 extern void main_loop(struct synth_t *s);
 extern void *queue_runner(void *arg);
 extern int init_audio(void);
+extern void lock_audio_mutex(void);
+extern void unlock_audio_mutex(void);
 extern volatile int stopped;
 
 extern espeak_AUDIO_OUTPUT audio_mode;
diff --git a/synth.c b/synth.c
index f7f13c3..e19d95e 100644
--- a/synth.c
+++ b/synth.c
@@ -114,7 +114,9 @@ espeak_ERROR set_volume(struct synth_t * s, int vol, enum adjust_t adj)
 
 espeak_ERROR stop_speech(void)
 {
+	lock_audio_mutex();
 	stopped = 1;
+	unlock_audio_mutex();
 	return (espeak_Cancel());
 }
 
@@ -122,7 +124,9 @@ espeak_ERROR speak_text(struct synth_t * s)
 {
 	espeak_ERROR rc;
 
+	lock_audio_mutex();
 	stopped = 0;
+	unlock_audio_mutex();
 	rc = espeak_Synth(s->buf, s->len + 1, 0, POS_CHARACTER, 0, 0, NULL,
 					  NULL);
 	return rc;