--- a/dom/media/webaudio/AudioNodeEngine.cpp
+++ b/dom/media/webaudio/AudioNodeEngine.cpp
@@ -74,18 +74,42 @@ void AudioBufferAddWithScale(const float
   if (mozilla::supports_neon()) {
     AudioBufferAddWithScale_NEON(aInput, aScale, aOutput, aSize);
     return;
   }
 #endif
 
 #ifdef USE_SSE2
   if (mozilla::supports_sse2()) {
-    AudioBufferAddWithScale_SSE(aInput, aScale, aOutput, aSize);
-    return;
+    if (aScale == 1.0f) {
+      while (aSize && (!IS_ALIGNED16(aInput) || !IS_ALIGNED16(aOutput))) {
+        *aOutput += *aInput;
+        ++aOutput;
+        ++aInput;
+        --aSize;
+      }
+    } else {
+      while (aSize && (!IS_ALIGNED16(aInput) || !IS_ALIGNED16(aOutput))) {
+        *aOutput += *aInput*aScale;
+        ++aOutput;
+        ++aInput;
+        --aSize;
+      }
+    }
+
+    // we need to round aSize down to the nearest multiple of 16
+    uint32_t alignedSize = aSize & ~0x0F;
+    if (alignedSize > 0) {
+      AudioBufferAddWithScale_SSE(aInput, aScale, aOutput, alignedSize);
+
+      // adjust parameters for use with scalar operations below
+      aInput += alignedSize;
+      aOutput += alignedSize;
+      aSize -= alignedSize;
+    }
   }
 #endif
 
   if (aScale == 1.0f) {
     for (uint32_t i = 0; i < aSize; ++i) {
       aOutput[i] += aInput[i];
     }
   } else {

--- a/dom/media/webaudio/blink/ReverbAccumulationBuffer.cpp
+++ b/dom/media/webaudio/blink/ReverbAccumulationBuffer.cpp
@@ -91,40 +91,20 @@ int ReverbAccumulationBuffer::accumulate
 
     float* destination = m_buffer.Elements();
 
     bool isSafe = writeIndex <= bufferLength && numberOfFrames1 + writeIndex <= bufferLength && numberOfFrames2 <= bufferLength;
     MOZ_ASSERT(isSafe);
     if (!isSafe)
         return 0;
 
-#ifdef USE_SSE2
-    // It is unlikely either the source is aligned or the number of values
-    // is a multiple of 16, so we just add them here rather than calling
-    // AudioBufferAddWithScale.
-    //
-    // TODO: Ideally we would use scalar calls when necessary and switch
-    //       to vector calls when we have aligned sources and destinations.
-    //       See Bug 1263910.
-    for (uint32_t i = 0; i < numberOfFrames1; ++i) {
-      destination[writeIndex + i] += source[i];
-    }
-
-    // Handle wrap-around if necessary.
-    if (numberOfFrames2 > 0) {
-        for (uint32_t i = 0; i < numberOfFrames2; ++i) {
-          destination[i] += source[numberOfFrames1 + i];
-        }
-    }
-#else
     AudioBufferAddWithScale(source, 1.0f, destination + writeIndex, numberOfFrames1);
     if (numberOfFrames2 > 0) {
         AudioBufferAddWithScale(source + numberOfFrames1, 1.0f, destination, numberOfFrames2);
     }
-#endif
 
     return writeIndex;
 }
 
 void ReverbAccumulationBuffer::reset()
 {
     PodZero(m_buffer.Elements(), m_buffer.Length());
     m_readIndex = 0;