Totally agree...not very audible even in a moderately noisy car as well, and because the speaker is in the back it can not be put into a holder, making it even more useless. I guess you could spend even more money and get a Bluetooth speaker of some sort.
I'm not happy with the volume level of the Nexus 4 in any app. Whether it's navigation, music, or even speakerphone. It's just far too low in my opinion.
I have found the volume to be brutal when plugged in to the audio aux jack as well as on some bluetooth devices. My car works fine with bluetooth as well as my Jambox but other devices are terrible. This is my one big gripe with the Nexus 4. Hope this is resolved in the Nexus 5.
Compared to the HTC Sensation 4G I had the Nexus 4 is like sitting in an orchestra–quite the boon when using Google Maps. I drive all over NYC in traffic with the window open but I do find occasion to close the window to listen to precise instructions... staying on the I-95 when I could have fared better on the I-87 makes me want to punch the roof! lol The Nexus 4 is nestled in a holster attached to the vent adjacent to my steering wheel so it's hard for me not to see it and hear it.
I'd look into a cheap FM transmitter to plug into the stereo minijack and tune the car radio into it.