Let me rephrase my original response.
The Left (driver side) front speaker's music level is reduced when the NAV announcement is made. The announcement is made only through that same speaker and not all of them. So if the music level is set very high I doubt if lowering the audio level in only the one speaker will help much in making the announcements audible. If the music audio level is set to a reasonable level the announcement through the single speaker is very audible with the announcement level set to mid-range.
YMMV