With Igo Nexgen Israel (NOT with Luna or other versions) if you use the internal TTS engine (folder "tts_nua_ve1" with the files of your preferred language, located in the main folder of Igo) the music of the car radio is lowered correctly when Igo speaks, and restored when he finishes speaking.

Prerequisite: the car radio must have the internal function "Navi remix" or something similar.
This works (at least for me) ONLY with Igo Nextgen Israel, with other versions of Igo Nextgen no, since they do not support the use of "tts_nua_ve1" to manage the TTS.