Merge pull request #2333 from birarda/master

fix for reversed MMX add in audio mixer
handle trivial case of not mixing silent audio streams
take an absolute value for correct loudness
clarify check for audio loudness in AudioMixer
Merge pull request #2336 from birarda/audio-scaling

handle trivial case of not mixing silent audio streams
Merge pull request #2342 from birarda/master

switch data for mix samples in AudioMixer
guard around update to last heard for a NULL matching node