It's actually not too difficult to make a linear fade out.
BUT
It is shit, because it's pretty fiddly and trying to get a logarithmic curve is difficult because I'm not that great at maths. So having tag fades with different attenuation curve types would be the best solution.
The whole audio system needs attention:
We NEED to be able to control the distance and roll-off factors for each sound / tag.
We need to be able to select which sounds / tags are effected by doppler, having all or nothing renders the feature practically useless.
We need to be able to apply effects to multiple tags.
This is basic stuff.