When someone describes their sound problem as intermittent, my mind immediately goes to the sound being interrupted by wind noise (via the intercom), CB, or weather alert. Each of these have their own on/off setting and volume control (as does the Navigation Prompts) and if they are on with the volume turned-down, it creates an "dead sound" interruption. Another possibility is you are inadvertently muting or turning the volume down on the Navigation Prompts.
If the Navigation Prompt option is switching on/off with no user interaction, then you have probably uncovered a bug in the release. To my knowledge, that problem does not exist in the US version, but I seem to recall it being a problem in a previous release.