When there are 2 audio tracks for a channel, swithing them will cause

[shark2010]

When there are 2 audio tracks for a channel, swithing them will cause picture to freeze for a while.

[Derrick]

..the reason is that the graph will rebuilt when a different audio track is selected. Apparently some other players like vlc use a different method which won't affect the video stream.

[Griga]

the reason is that the graph will rebuilt when a different audio track is selected.

Not necessarily, or only part of it, depending on whether the audio track change implies a format change (e.g. AC3 <-> MPEG Audio) or not. But at least playback must be stopped temporarily, because most DirectShow filters don't support dynamic stream switching in run state.