I have the exact same problem. What I do is use a non-standard music player like QuasarMX (http://katastrophos.net/quasarmx/) where you can manually crank up the music volume above 100%. It's not perfect because it can distort the sound (but QuasarMX is very good at avoiding it!), but for a long trip, it's an improvement.

There is no option in the Maps/Drive UI to do that in Harmattan, which is a shame because I think the option was there on Symbian. Maybe it's one of those options that exist but has no UI... (like automatic day/night mode and the time left/time of arrival/distance left switch).

If someone has another solution , I'd like to hear it too.