I'm using Haali Splitter.
Uploaded sample here (about 60Mb):
http://rapidshare.com/files/189166085/sample.mkv
File has 3 audiostreams: Eng/Rus/English commentary.
In mkvmerge I checked Eng - default track, but in BS defaut is Rus, don't know why (for example, in Media Player Classic, default is english in this sample).
On Shift+S changing streams 1 and 2, never 3.