removed SIMD instructions in AudioMixer; removed IsNaN check for _nextOutputTrailingLoudness

Issue: various float variables used in the AudioMixer were randomly becoming NaN.  The cause of this seems to be the use of __m64 variables and SIMD instructions.  When running dancer.js on a script agent (with local domain server and audiomixer), the script's audio is either heavily distorted or non-existent.

Fix: replaced the SIMD instructions with regular add instructions.  Also, since this seemed to fix the NaN issue, the NaN check that was added for _nextOutputTrailingLoudness in a previous PR will be removed.

Test: dancer.js audio is now working.  Additionally, sound effects from button presses on script overlays now works every time on my machine as opposed to maybe 25% of the time.