How is volume used on other platforms?
XNA says this:
Volume adjustment is based on a decibel, not multiplicative, scale. For example, when the
device volume is half of maximum (about 7 [this has been written in the old era with the 15 scale, now we have a 30 scale] in the Windows Phone user
interface), setting Volume to 0.6f or less is silent or nearly so, not volume 4
as you would expect from a multiplicative adjustment.
Setting Volume to 0.0 subtracts 96 dB from the volume. Setting Volume to 1.0 subtracts 0 dB from the volume. Values in between 0.0f and 1.0f subtract dB from the volume proportionally.
MediaElement uses a linear scale, but our perception of volume is logarithmic, hence the volume setting has to be exponential. On WP8 a volume of 0.85 seems to refer to 50% volume. I assume this because MSDN says that the default volume is 0.5, except on WP where it's 0.85 (and to my ears 0.85=50% volume seems to be correct).
Therefore a good approximation could be to use the 4th root of volume (0.5^(1/4)~0.84). I can submit a pull request for this on GitHub in case you want to test it without writing the code yourself