I don't have a Windows 7 machine to test it, but provided you installed the Cool VL Viewer properly, there is no reason why voice won't work...
Try this:
- Uninstall the viewer (make sure to backup any log you'd like to keep first).
- Install again the official Linden Lab viewer v1.23.5
- Check that voice works within it. If it doesn't work, then first resolve your problem (you can even use LL's ticket system if needed).
- Once voice works with the official viewer, install the Cool VL Viewer v1.23.5 over it (i.e in the same directory).
For the above steps, please also refer to the
Windows Installation Howto