use floating point decoders for mp1, mp2, mp3

[withmorten]

For some reason ffmpeg doesn't use the floating point decoders for mp1, mp2 and mp3 by default - this forces use of these.

Makes the spek not have all the "clipping" artifacts anymore - left with floating point decoders, right without:

http://i.imgur.com/DxnNBwM.jpg