There's a fix in the works to improve the audio. The main problem is the speaker.
The audio can be intercepted at pins 5&6 on the 15 pin green connector to the nexus box. The wires are white/red and white/black. That signal at full strength will probably overload an aux input, but could be brought into line with a simple resistance divider (I think). Or, just take the existing signal and use a better external speaker.
Btw: the volume control for the alert audio is found under 'airspace'. Even at max volume, you may still find it a little marginal, depending on how you have your nexus box mounted.