you have switched your windows 7 output to Digital Output.
Click the START symbol (bottom row left sided)
selec DEVICES/PRINTER click
opend box shows you in first row Printer
in second row your PC
set the cursor above PC and click the RIGHT mouse button
a box would be opend where it shws which output chnnel is selected.
i bet the DIGITAL OUTPUT is on
go to row (SPEAKER) and if the cursor is above click the right mouse button again
a table of selections are opend. switch the ACTIVATE selecton and the sound comes back.
a half hour before i had the same problem, while i try to install my ADS radio.