There are three issues with MediaBrowser search under "Audio/Online Sources".
It is not displaying all the hits. (A searched for "Jazz" and it displayed only 12 hits when there were over twenty). Same with other searches.
There is a problem with the matching as a seach for "classical" wrongly matched with "classic" (resulting in including classic blues and classic rock).
The search is not restricted to the section you are in i.e. if you search from within Audio/Online Sources it produces results from Albums, Tracks, etc. (that are not in Online Sources in addition to the Online Sources). These results might have been valid if the search was at a higher level.
see Forum/Serviio Support & Help http://forum.serviio.org/viewtopic.php?f=5&t=24435