I had the exact same problem with my Toshiba L755 after upgrading from Win 7 to Win 10. Here's what worked for me:
- Go to Device Manager in Control Panel
- Expand "Sound, video and game controllers"
- Right-click on Conexant SmartAudio HD and select "Update Driver Software"
- Select "Browse my computer for driver software"
- Select "Let me pick from a list of device drivers on my computer"
- Under "Model," select "High Definition Audio Device" and click "Next." Ignore the "Update Driver Warning" by clicking "Yes"
You should get a message that the driver was successfully installed, and your sound should be back. Too bad Toshiba arbitrarily decided not to bother supporting the L755 series for upgrades to Win 10.