--- a/dom/media/CubebUtils.cpp
+++ b/dom/media/CubebUtils.cpp
@@ -195,27 +195,27 @@ void PrefChanged(const char* aPref, void
     Preferences::GetCString(aPref, value);
     StaticMutexAutoLock lock(sMutex);
     if (value.IsEmpty()) {
       sVolumeScale = 1.0;
     } else {
       sVolumeScale = std::max<double>(0, PR_strtod(value.get(), nullptr));
     }
   } else if (strcmp(aPref, PREF_CUBEB_LATENCY_PLAYBACK) == 0) {
+    StaticMutexAutoLock lock(sMutex);
     // Arbitrary default stream latency of 100ms.  The higher this
     // value, the longer stream volume changes will take to become
     // audible.
     sCubebPlaybackLatencyPrefSet = Preferences::HasUserValue(aPref);
     uint32_t value = Preferences::GetUint(aPref, CUBEB_NORMAL_LATENCY_MS);
-    StaticMutexAutoLock lock(sMutex);
     sCubebPlaybackLatencyInMilliseconds = std::min<uint32_t>(std::max<uint32_t>(value, 1), 1000);
   } else if (strcmp(aPref, PREF_CUBEB_LATENCY_MSG) == 0) {
+    StaticMutexAutoLock lock(sMutex);
     sCubebMSGLatencyPrefSet = Preferences::HasUserValue(aPref);
     uint32_t value = Preferences::GetUint(aPref, CUBEB_NORMAL_LATENCY_FRAMES);
-    StaticMutexAutoLock lock(sMutex);
     // 128 is the block size for the Web Audio API, which limits how low the
     // latency can be here.
     // We don't want to limit the upper limit too much, so that people can
     // experiment.
     sCubebMSGLatencyInFrames = std::min<uint32_t>(std::max<uint32_t>(value, 128), 1e6);
   } else if (strcmp(aPref, PREF_CUBEB_FORCE_SAMPLE_RATE) == 0) {
     StaticMutexAutoLock lock(sMutex);
     sCubebForcedSampleRate = Preferences::GetUint(aPref);
@@ -229,16 +229,17 @@ void PrefChanged(const char* aPref, void
     } else if (value.EqualsLiteral("normal")) {
       cubeb_set_log_callback(CUBEB_LOG_NORMAL, CubebLogCallback);
       cubebLog->SetLevel(LogLevel::Error);
     } else if (value.IsEmpty()) {
       cubeb_set_log_callback(CUBEB_LOG_DISABLED, nullptr);
       cubebLog->SetLevel(LogLevel::Disabled);
     }
   } else if (strcmp(aPref, PREF_CUBEB_BACKEND) == 0) {
+    StaticMutexAutoLock lock(sMutex);
     nsAutoCString value;
     Preferences::GetCString(aPref, value);
     if (value.IsEmpty()) {
       sCubebBackendName = nullptr;
     } else {
       sCubebBackendName = new char[value.Length() + 1];
       PodCopy(sCubebBackendName.get(), value.get(), value.Length());
       sCubebBackendName[value.Length()] = 0;