You did not say if you had made any changes to the system. If the system did that out of the box, you would return it to Dell.
Without the details it is guess work from here. The sound problem with Windows 7 and recent Dell models is with the drivers. You have to use the Dell drivers, not the drivers from the chip maker. But this is only a problem for individuals who decide to reformat the drive and start over. They forget that the drivers are not on the install DVD. Generic drivers are not quite right.
The above is all guess work. Maybe it is wrong. But without specific details of how and when this started, the is about the best. The guess work is based on similar post on the Dell forums. The typical solution is to download the exact drivers from Dell for that model.
Even if device manager says all is OK.
And if is just not Windows 7. The rule also works for older Dell machines with XP. Dell does their own drivers. Get the Dell drivers.
If anybody else here knows more about this, please come in.