Not using voladj - voladj is effectively a form of compression, compressing the dynamic range of a piece of music so that quiet passages appear louder and vice versa.

But in theory, GPSapp could talk to hijacks mixer IOCTLs to adjust the volume.
A couple of issues to consider though;
GPSapp doesn't currently do anything whilst it is not active. So you'd have to have GPSapp visible in order for speed adjusment to work. I think that Jan is planning to address this somehow.
How to track/account for intentional player volume changes? One approach would be for GPSapp to try and track what the player is setting the volume too and to modify it from there based upon current speed. The other approach would be for GPSapp to simply read and modify the volume every time the speed changes past some boundary. Both options have their merits and pitfalls. I'd suggest the 2nd is probably easier to deal with.
Another issue is to ensure that the system had some hysterysis built in - we wouldn't want the volume oscillating because we're cruising around a boundary point.
And finally, does anyone have any idea how much adjustment would be desirable? And where would you start? 0.5dB/10mph above 30mph? 1dB/10mph?