Iphone And Sat Nav

[sddsddean]

I think the answer to this going to be 'no', but I'll ask anyway!
I've got an iPhone 4 and have downloaded a free sat nav to it (nav free by geolife). Is there any way to 'force' the commentary to be transmitted to my bluetooth headset if it is not an option in the sat nav's menu? I don't think there is, but thought I'd ask as its a bit too noisy in the truck to hear the commentary.