Apple Car Play Question Volume Control

When you set up a route, there is an option to use the speaker only for alerts, not directions. I would guess that that option is what the yellow icon is for.

 

On mine, I have a volume rocker on the left side of the steering wheel. I found that holding the "source" button also mutes the sound.

 

Is the icon on your phone or the cars infotainment system?

Found this with google:

"

• volume icon: turns on all voice guidance
• volume icon with a triangle containing an exclamation mark: turns on voice alerts only
• volume icon crossed out: turns off all voice guidance

Note: "Voice alerts only" mode only gives voice notifications of suggested route changes or traffic issues coming up on your current route.
Also, with any of these options, Maps still shows the route you've requested. These volume icons only control whether Siri speaks (ie, provides voice guidance for) the turn-by-turn steps through the route."​

Original Link

Not really related to your question, but something that might be useful to others:

I've found that sometimes the navigation volume is much lower than the music volume. To fix it I've been going into the infotainment system menus to turn it up, but my wife discovered another way.
While the navigation is speaking use the volume rocker on the steering wheel to turn it up/down. Somehow the car knows to only increase/decrease the navigation volume independently from the music.
Sooo much easier than navigating the infotainment while driving!!