diff --git a/content/media/AudioStream.cpp b/content/media/AudioStream.cpp
index 9e02f77cb2df..0955272938a7 100644
--- a/content/media/AudioStream.cpp
+++ b/content/media/AudioStream.cpp
@@ -76,6 +76,15 @@ bool AudioStream::sCubebLatencyPrefSet;
   return GetCubebContextUnlocked();
 }
 
+/*static*/ void AudioStream::InitPreferredSampleRate()
+{
+  StaticMutexAutoLock lock(sMutex);
+  if (sPreferredSampleRate != 0 ||
+      cubeb_get_preferred_sample_rate(GetCubebContextUnlocked(), &sPreferredSampleRate) != CUBEB_OK) {
+    sPreferredSampleRate = 44100;
+  }
+}
+
 /*static*/ cubeb* AudioStream::GetCubebContextUnlocked()
 {
   sMutex.AssertCurrentThreadOwns();
@@ -266,25 +275,8 @@ int64_t AudioStream::GetWritten()
 
 /*static*/ int AudioStream::PreferredSampleRate()
 {
-  const int fallbackSampleRate = 44100;
-  StaticMutexAutoLock lock(sMutex);
-  if (sPreferredSampleRate != 0) {
-    return sPreferredSampleRate;
-  }
-
-  cubeb* cubebContext = GetCubebContextUnlocked();
-  if (!cubebContext) {
-    sPreferredSampleRate = fallbackSampleRate;
-  }
-  // Get the preferred samplerate for this platform, or fallback to something
-  // sensible if we fail. We cache the value, because this might be accessed
-  // often, and the complexity of the function call below depends on the
-  // backend used.
-  if (cubeb_get_preferred_sample_rate(cubebContext,
-                                      &sPreferredSampleRate) != CUBEB_OK) {
-    sPreferredSampleRate = fallbackSampleRate;
-  }
-
+  MOZ_ASSERT(sPreferredSampleRate,
+             "sPreferredSampleRate has not been initialized!");
   return sPreferredSampleRate;
 }
 
diff --git a/content/media/AudioStream.h b/content/media/AudioStream.h
index fc040020c32a..2739de44082c 100644
--- a/content/media/AudioStream.h
+++ b/content/media/AudioStream.h
@@ -179,8 +179,11 @@ public:
   // Returns the maximum number of channels supported by the audio hardware.
   static int MaxNumberOfChannels();
 
-  // Returns the samplerate the systems prefer, because it is the
-  // samplerate the hardware/mixer supports.
+  // Queries the samplerate the hardware/mixer runs at, and stores it.
+  // Can be called on any thread. When this returns, it is safe to call
+  // PreferredSampleRate without locking.
+  static void InitPreferredSampleRate();
+  // Get the aformentionned sample rate. Does not lock.
   static int PreferredSampleRate();
 
   AudioStream();
diff --git a/content/media/MediaStreamGraph.cpp b/content/media/MediaStreamGraph.cpp
index 671db6eeaf2f..db8fd8c577c3 100644
--- a/content/media/MediaStreamGraph.cpp
+++ b/content/media/MediaStreamGraph.cpp
@@ -1328,6 +1328,12 @@ MediaStreamGraphImpl::ForceShutDown()
   }
 }
 
+void
+MediaStreamGraphImpl::Init()
+{
+  AudioStream::InitPreferredSampleRate();
+}
+
 namespace {
 
 class MediaStreamGraphInitThreadRunnable : public nsRunnable {
@@ -1340,6 +1346,7 @@ public:
   {
     char aLocal;
     profiler_register_thread("MediaStreamGraph", &aLocal);
+    mGraph->Init();
     mGraph->RunThread();
     return NS_OK;
   }
diff --git a/content/media/MediaStreamGraphImpl.h b/content/media/MediaStreamGraphImpl.h
index c8cd27a8b1be..76b036e1c4df 100644
--- a/content/media/MediaStreamGraphImpl.h
+++ b/content/media/MediaStreamGraphImpl.h
@@ -150,6 +150,10 @@ public:
    */
   void ShutdownThreads();
 
+  /**
+   * Called before the thread runs.
+   */
+  void Init();
   // The following methods run on the graph thread (or possibly the main thread if
   // mLifecycleState > LIFECYCLE_RUNNING)
   /**
diff --git a/content/media/webaudio/AudioContext.cpp b/content/media/webaudio/AudioContext.cpp
index c757d574a6b4..f269163652f7 100644
--- a/content/media/webaudio/AudioContext.cpp
+++ b/content/media/webaudio/AudioContext.cpp
@@ -34,6 +34,7 @@
 #include "ConvolverNode.h"
 #include "OscillatorNode.h"
 #include "nsNetUtil.h"
+#include "AudioStream.h"
 
 namespace mozilla {
 namespace dom {
@@ -65,12 +66,22 @@ NS_INTERFACE_MAP_END_INHERITING(nsDOMEventTargetHelper)
 
 static uint8_t gWebAudioOutputKey;
 
+float GetSampleRateForAudioContext(bool aIsOffline, float aSampleRate)
+{
+  if (aIsOffline) {
+    return aSampleRate;
+  } else {
+    AudioStream::InitPreferredSampleRate();
+    return static_cast<float>(AudioStream::PreferredSampleRate());
+  }
+}
+
 AudioContext::AudioContext(nsPIDOMWindow* aWindow,
                            bool aIsOffline,
                            uint32_t aNumberOfChannels,
                            uint32_t aLength,
                            float aSampleRate)
-  : mSampleRate(aIsOffline ? aSampleRate : IdealAudioRate())
+  : mSampleRate(GetSampleRateForAudioContext(aIsOffline, aSampleRate))
   , mNumberOfChannels(aNumberOfChannels)
   , mIsOffline(aIsOffline)
   , mIsStarted(!aIsOffline)