This has happened to me twice before. I do not remember if it was before or after I did the 29.5 Radio Update or not on my 2015 SS.
I was listening to XM or USB MP3's at the time, and either changed channel or song or something, then all of a sudden all sound muted. I thought I hit the Mute button, tried volume control, did not get my audio back. Then I noticed the blinkers did not work as well, and noticed the blinker audio normally comes out of speakers. Backup sensor "Beeping" also did not work, and I assume collision alert also. Radio continued to play what it was playing, just no audio.
The fix for this for me was leaving the car off for about 15-30 minutes. A power off / open door / power on did not do it. This allowed for the radio to completely reset/reboot. Getting out of the car/turning off the radio is not enough. The radio continues to remain powered by the car for 15-30 minutes after exiting the vehicle. You will notice this in other cases, when you power on the vehicle after it being off longer than 15-30 minutes, GPS Navigation will say "Initiating" for a good 1-2 minutes before being usable, where as if you just turned the car off and got back in, ex a quick stop at the store or for gas, it will turn back on right away and be usable.
This is a legitimate safety concern, as it disabled feedback for blinker, disabled collision alert, disabled backup and parking sensor feedback. If the user had the radio not playing music, or at a low volume, the driver may not even notice the lack of sound, and could be an issue if they rely on the alerts.