I have a 96 GMC Yukon and blinker audio does not shut off after a turn. Visually it works as it should but the sound continues sometimes for weeks at a time. Starts as soon as car is started. But as I said it works and the turn signals do not flash even though sound is heard.
I had the same problem with my 2000 Blazer, it was the multifunction switch (turn signal, cruise, wiper, etc). You can try spraying electrical contact cleaner/WD40 into the switch to clean the contacts. I ended up replacing mine. Figure around $200 for the part and a couple of hours of labor.