I have now solved my problem.

Thanks to "Midnight Blues" for his reply.

It appears that the "sys.txt" input that he suggested only worked for Android systems up to 4.44, after that this requirement is included in "settings".

If you go to "Factory Settings/Voices" you can set the individual voice levels seperately. If you set the system voice to,say, +20 and the other voice levels to +10 then the navigation notifications are superimposed on the system voice, radio, music etc, and are perfectly audible. Play with these settings to get the sound levels that suit you.

I shall now continue to use iGO Primo 2.4 as i like it better than Nextgen. If anyone knows how to receive the weather on Primo i would appreciate it.