iGuidance TTS audio coming through speaker, not A2DP?

[zzed]

Up front details:

• Verizon's version of HTC Touch Pro (XV6850)
• Verizon stock ROM MR1 (2009-06-11)
• Connects to Sony MEX-BT5000 head unit for hands-free and A2DP

I recently installed iGuidance 2009 on my TP, mainly because it has text-to-speech directions (as compared to Google Maps, which doesn't). It works great, except for one detail. When connected to the head unit in my truck, the audio for "In 500ft, turn right on" is played through the truck speakers, but when the name of the street is said, that audio is played through the TP speaker. Any ideas why this happens and/or how to make all of the iGuidance audio play through the stereo?

-=Zzed=-