The only sound problem I've been seeing with 7 since the Vista and now 7 64bit application and driver updates came out from Creative has been with the custom assigned Windows logon sound with the default startup sound unchecked.
That still won't play as the desktop is reached. But that seems to be Windows 64bit related since you will hear it on the 32bit RC.
For onboard or embedded audio the drivers are being refreshed and made active again once Windows is rebooted there. Apparently you have something running maybe in the background that occasionally clashes with the sound drivers and knocks them oiut by trying to access the same resources.
XP used to see that often when a separate sound card was installed on the first pci slot due to a shared resource problem prior to SP2. You'll need to look over the startups as well as various processes to see if this is not strictly a Windows/device driver related problem.