AIUI, the Roku Soundbridge clients will tend to use any media Server's Search facility to generate the browse hierarchy. When the Roku clients do this, they generate the nice letters on the right.
The Twonky server deliberately turns the Search function off for Roku clients. The advantage of this policy is that the server will force these clients to use the clever hierarchy of server containers. The disadvantages are that you lose the client-generated nice letters, that the search button doesn't work, and (of course) you can't perform searches.
In the Soundbridge software (at least for version 2.7, released March 2007 ), when the client-generated containers are used, at the bottom of the list of these containers (Album, Artist, etc.) there is a 'Server Containers' container, which gets you into the clever hierarchy on the Twonky server. So suppressing search on the Roku clients seems rather unnecessary - you can see the server containers if you want, just not by default.
In Twonky version 4.2 beta 3, there was an option for 'disable search for Roku clients'. You could clear this checkbox, (restart the twonky server) and voilÃƒÂ , you use the Soundbridge-generated hierarchy, and the letters appear. And the search would work, too.
Unfortunately, in version 4.2 RC2 of the server, it seems this facility for suppressing the Roku client behaviour has been removed. And even when you over-ride the Client detection, and pretend the client is a Generic DNLA client, you still have Search suppressed. This either suggests the Client detection over-ride is broken, or the search function is broken.