There should be a Overwolf icon in your taskbar (top) of your TS. Click on it, it will start (or install Overwolf if you haven't got it). Once you've done that it should start to display the speakers in one of the corners.
If that doesn't work, you probably need to enable your notification. To do so, you just click on the TS app icon. Then click the wrench for settings and enable notifications and which corner you want them to be displayed in. Done.
Hope this helps you out, if not let me know. I'm currently using Overwolf whenever i play with guys who i can't recognize by their voice