I have here two models of T420s running Windows 7 and Windows 8.
The Windows 7 model is quiet and the service needs no CPU time during Idle.
The Windows 8 model is noisy (the fan is spinning) and the Conexant Audio Message Service (Version 220.127.116.11) consumes 30% of CPU during Idle.
A community users tested and found this workaround - disable the Conexant Audio service.
1) At the Windows 8 Start screen, type "Services". 2) Click "Services" (its icon looks like two silver-blue gears). 3) Scroll down to "Conexant Audio Message Service" and double-click it 4) Change the Startup Type to "Disabled" 5) Click the "Stop" button, then click OK and close the Services window