This is not a bug. This is the result of logging in on different grids which got different sounds than in SL, but with the same UUID as in SL. This affects all viewers.
Since the viewer does not include the UI sounds in its UI files, it fetches the UI sounds from the asset server instead, unless the sound is already cached (in the saved VFS file), in which case it uses the cached sound.
The problem is that if you log into another grid and use the UI, then relog in SL, the UI sounds corresponding to the ones you got during the OpenSim session are probably still cached, and the viewer will use those cached sounds, thus mixing the UI sounds from different grids.
I produced a patch that allows to use UI sounds that would be bundled with the viewer, as part of its UI files, and asked LL if TPV developers could reuse and distribute the UI sounds fetched from the SL grid. The answer was no (see
this thread in the opensource-devel list).
Still the patch is applied in the current Cool VL Viewer release (see
this announcement), and you can therefore get rid of those problems once and for all by yourself, by copying (while the viewer runs and is logged into SL) the decoded UI sound (the list is
available here) from the viewer cache folder into a "sounds" folder in the skins/default/ directory of the viewer installation. Once this is done, the viewer will always fetch the UI sounds from its skin directory (skins/default/sounds/) instead of fetching them from the server or from the cache, thus getting rid of this issue.