Windows 7 lowering volume without my consent

Sometimes when several system sounds play at once, Windows 7 will forcibly lower their volume even though I have disabled automatic volume reduction in Sound -> Communications.

In my case, I have sounds set to play when a program opens or closes. Whenever a UAC prompt appears it often causes the issue because it plays program start, UAC notification, and program stop sounds all at once. I never use any kind of audio communication on my computer so I can only guess this is something else. I have seen this problem occasionally for years, across different hardware. Is there any known tweak/hack for stopping this or any confirmation that it can't be fixed?

I found a way to fix the UAC issue specifically:

1. Open the Registry Editor. (See here for information if you don't know how to edit the registry.)
2. Open the HKEY_USERS key. (Keys have folder icons by them.)
3. Open the first key in the list and go to Software\Microsoft\Multimedia\Audio. The Multimedia and Audio keys probably won't exist so create them by right clicking on the key to place them in and selecting New -> Key.
4. Make sure the Audio key is selected and check the right pane to see if UserDuckingPreference exists. If not, create it by right clicking in the pane and selecting New -> DWORD.
5. Double-click on UserDuckingPreference and set its value to 3. (The Hexa/decimal option doesn't matter in this case.)
6. Go back up to HKEY_USERS and close the keys you had open under it, then start over from step 3 for each of the other top keys in HKEY_USERS.

This will force the "Do nothing" setting on every user account in the system, including the special account that is used for the UAC dim screen.

This probably fixes other potential issues with special screens like the logon screen.