Fixes.

effect_stereo_mono.cpp

@@ -49,8 +49,10 @@ void AudioEffectStereoMono::update(void)
 
       for (uint16_t i = 0; i < AUDIO_BLOCK_SAMPLES; i++)
       {
-        *bp[0]++ = *bp[0] >> 1 + *bp[1] >> 1;
+        *bp[0] = (*bp[0] >> 1) + (*bp[1] >> 1);
-        *bp[1]++ = *bp[0];
+        *bp[1] = *bp[0];
+        bp[0]++;
+        bp[1]++;
       }
     }
   }