yahoo messenger and sound and different accounts

I've got yahoo messenger installed.
And there are two accounts on my system.
Both accounts are normal user accounts.
BTW running vista 64bit edition.
In one account yahoo messenger works fine.
In the other account yahoo messenger cannot make any sounds.
(I've already found the special volume control for yahoo messenger and it is
set to max.)

I used process explorer from sysinternals.com.
I created two log files running yahoo messenger in both accounts and
compared the log files.
As it turns out, there is some registry key which is available in one
account but not in the other:

HKEY_USERS\S-1-5-21-3269138072-1672139622-993279305-1001_Classes\Wow6432Node\CLSID\{3050F406-98B5-11CF-BB82-00AA00BDCE0B}

This has something to do with Internet Explorer and processing of html.

Anybody any clue?