Head scratching here too...
Just tried the KDE 2012.12 iso under my VirtualBox 4.2.6 and boots to live session with no issues.
Just reposting my settings for easy reference:GENERAL
Operating system: Mandriva or Other LinuxSYSTEMMotherboard
Base Memory: 1024MB
Extended features: Enable IO APICProcessor
Extended Features: Enable PAE/NXAcceleration
Hardware Virtualization: Enable VT-x/AMD-V, Enable Nested PagingDISPLAYVideo
Video Memory: 128MB
Monitor Count: 1
Extended Features: Enable 3D Acceleration
Remote Display: Disabled.STORAGE
Normal (VDI), 20GB, Fixed size storage.AUDIO
Host Audio Driver: Windows DirectSound
Audio Controller: ICH AC97
Maybe you should try with the iso directly without using your dvd drive?
Have you tried enabling/disabling some options?
Has your virtual disk a fixed size or is it dynamically allocated?