I now also have the same irritating problem, here's my setup:
n97 rm-505 0585262
ovi maps v3.04.
1) I have the music playing over the fm-transmitter, primarily to prevent the fm transmitter from timing out and switching itself off, which is another annoyance.
2) I then flash up ovi-maps, as soon as ovi-maps plays any sound, whether voice or safety camera warnings etc, it plays ALL sound through the phone speakers including music, whilst the car stereo goes completely quiet.
This phone is perfectly capable of running both satnav sound and music simultaneously over the fm transmitter, and must be a software issue, but is a fix within the capabilities of a user or do we (joe bloggs in the street) have to wait for a software update either through ovi-maps or the phone firmware?
If it's the latter, then i'll be withdrawing a pension when it happens, as my n97 handset is vodafone branded (I & many others have been waiting for 5 months for them to finish wrecking nokias update), and i'm very reluctant to risk bricking it through debranding. Vodafone must either have their priorities elsewhere, or they need a fire lit under their **bleep**.