Audio: Reintroduce Boost Volume on downmix

After reading this forum thread here: http://forum.xbmc.org/showthread.php?tid=196421&pid=1801222 I came to the conclucsion that though normalizelevels is fully correct, that users want to think with the inverse, e.g. boost volume on downmix.

This PR adjusts the code and inverts the flags in the relevant parts of the code.

Also the description - as correct it was - does not help the average joe much according to the above thread.