resolve looplength bug in xaudio

Xaudio2 requires loop length in samples and takes channels into account so existing was factor of 2 out in mono, factor of 4 out in stereo. Seems the new SoundEffect(byte[] overloads were the only ones effected (fromStream already had correct values)
Merge pull request #3287 from madninjaskillz/develop

Resolve looplength bug in xaudio