No Sound - Audio service is not running

Hello, I have a Dell Inspiron M5010 (Windows 7)

Yesterday my speakers stopped working. Actually I think that they are fine because I have connected the computer with external speakers that are 100% working this morning and still there was no sound.

I've tried some solutions given by other people in the web but unfortunately sound does not return.

For instance, I've done a system restore, re-install drivers, format the laptop, open services.msc and restarted the Windows audio but the outcome was always the same.

Since it is within warranty, I will take it to the retailler soon if I can't solve it and I hope that I don't have to pay anything.

I am looking forward to read your suggestions.

Thanks for your help.
I've downloaded the driver in the link you gave me but the problem still exists.
What else can I try next?

No, nobody muted the sound manually.

Thanks anyway.