The 10 second clip plays fine (audio and video) here with QuickTime Player 7, MPEG Streamclip, and VLC. QuickTime 10 behaves as it does on your computer, but that is normal for AC-3 files.
I have Perian 1.2.3
The Perian version of "A52Codec.component" in:
/Library/Audio/Plug-Ins/Components/
has been replaced with the special Mountain Lion version. It is 194 KB. The Perian version is much larger.
The only other file in that folder is "Flip4Mac WMA Import.component"
I do not have a "A52Codec.component" in:
/Library/QuickTime/
but it does contain the Perian files "AC3MovieImport.component" and "Perian.component".
Did you remove the obsolete QuickTime Components: FFusion, Xvid Delegate, 3ivX, DivX, DivX Decoder, XviD, msmpeg4v1, msmpeg4v2, AviImporter, EX_M4S2, Casio AVI Importer, AC3 Codec, and MatroskaQT? See:
<http://perian.org>
(Scroll down to the Installation section.)