This is a known issue.
In Cubase SX, you can select Buffer Sizes that are incompatible with Digidesign hardware (Digi 002, Digi 002 Rack, Digi 001, Mbox). Selecting incompatible Buffer Sizes results in distorted sound and Cubase possibly quitting.
Note: Cubase SX launches with a default Buffer Size of 256. Because this Buffer Size is not supported by Mbox, it must be changed before attempting to use Mbox for audio output.
To change the Buffer Size in Cubase SX:
1) Choose Devices > VST Multitracks.
2) Select a Buffer Size that is supported by your Digidesign hardware. Digidesign hardware supports different Buffer Sizes, as follows:
Digi 002 and Digi 002 Rack: 64, 128, 256, 512, 1024 Buffer Sizes.
Digi 001: 128, 256, 512, 1024 Buffer Sizes.
Mbox: 512, 1024 Buffer Sizes.