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diff --git a/dom/media/AudioMixer.h b/dom/media/AudioMixer.h
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+++ b/dom/media/AudioMixer.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef MOZILLA_AUDIOMIXER_H_
+#define MOZILLA_AUDIOMIXER_H_
+
+#include "AudioSampleFormat.h"
+#include "AudioSegment.h"
+#include "AudioStream.h"
+#include "nsTArray.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/PodOperations.h"
+
+namespace mozilla {
+
+struct MixerCallbackReceiver {
+  // MixerCallback MAY modify aMixedBuffer but MUST clear
+  // aMixedBuffer->mBuffer if its data is to live longer than the duration of
+  // the callback.
+  virtual void MixerCallback(AudioChunk* aMixedBuffer,
+                             uint32_t aSampleRate) = 0;
+};
+/**
+ * This class mixes multiple streams of audio together to output a single audio
+ * stream.
+ *
+ * AudioMixer::Mix is to be called repeatedly with buffers that have the same
+ * length, sample rate, sample format and channel count. This class works with
+ * planar buffers.
+ *
+ * When all the tracks have been mixed, calling MixedChunk() will provide
+ * a buffer containing the mixed audio data.
+ *
+ * This class is not thread safe.
+ */
+class AudioMixer {
+ public:
+  AudioMixer() { mChunk.mBufferFormat = AUDIO_OUTPUT_FORMAT; }
+
+  ~AudioMixer() = default;
+
+  void StartMixing() {
+    mChunk.mDuration = 0;
+    mSampleRate = 0;
+  }
+
+  /* Get the data from the mixer. This is supposed to be called when all the
+   * tracks have been mixed in. The caller MAY modify the chunk but MUST clear
+   * mBuffer if its data needs to survive the next call to Mix(). */
+  AudioChunk* MixedChunk() {
+    MOZ_ASSERT(mSampleRate, "Mix not called for this cycle?");
+    mSampleRate = 0;
+    return &mChunk;
+  };
+
+  /* Add a buffer to the mix. The buffer can be null if there's nothing to mix
+   * but the callback is still needed. */
+  void Mix(AudioDataValue* aSamples, uint32_t aChannels, uint32_t aFrames,
+           uint32_t aSampleRate) {
+    if (!mChunk.mDuration) {
+      mChunk.mDuration = aFrames;
+      MOZ_ASSERT(aChannels > 0);
+      mChunk.mChannelData.SetLength(aChannels);
+      mSampleRate = aSampleRate;
+      EnsureCapacityAndSilence();
+    }
+
+    MOZ_ASSERT(aFrames == mChunk.mDuration);
+    MOZ_ASSERT(aChannels == mChunk.ChannelCount());
+    MOZ_ASSERT(aSampleRate == mSampleRate);
+
+    if (!aSamples) {
+      return;
+    }
+
+    for (uint32_t i = 0; i < aFrames * aChannels; i++) {
+      mChunk.ChannelDataForWrite<AudioDataValue>(0)[i] += aSamples[i];
+    }
+  }
+
+ private:
+  void EnsureCapacityAndSilence() {
+    uint32_t sampleCount = mChunk.mDuration * mChunk.ChannelCount();
+    if (!mChunk.mBuffer || sampleCount > mSampleCapacity) {
+      CheckedInt<size_t> bufferSize(sizeof(AudioDataValue));
+      bufferSize *= sampleCount;
+      mChunk.mBuffer = SharedBuffer::Create(bufferSize);
+      mSampleCapacity = sampleCount;
+    }
+    MOZ_ASSERT(!mChunk.mBuffer->IsShared());
+    mChunk.mChannelData[0] =
+        static_cast<SharedBuffer*>(mChunk.mBuffer.get())->Data();
+    for (size_t i = 1; i < mChunk.ChannelCount(); ++i) {
+      mChunk.mChannelData[i] =
+          mChunk.ChannelData<AudioDataValue>()[0] + i * mChunk.mDuration;
+    }
+    PodZero(mChunk.ChannelDataForWrite<AudioDataValue>(0), sampleCount);
+  }
+
+  /* Buffer containing the mixed audio data. */
+  AudioChunk mChunk;
+  /* Size allocated for mChunk.mBuffer. */
+  uint32_t mSampleCapacity = 0;
+  /* Sample rate the of the mixed data. */
+  uint32_t mSampleRate = 0;
+};
+
+}  // namespace mozilla
+
+#endif  // MOZILLA_AUDIOMIXER_H_