audio/utils: change_bps: add dithering

By reducing bit-depth of audio, quantization noise is introduced. If the
bit depth is reduced by consistent rounding (either always down, always
up, or always to nearest) the quantization noise is correlated with the
input signal and percieved as distortion. Adding a small amount of noise
to the input signal before reducing the bit-depth causes random
rounding, which makes the noise uncorrelated and uniformly spread
through the whole spectrum.