Either your computer or your sound card may not be fast enough.

Remember, AC3 is extremely high quality, 6 channel sound. If you desperately need to hear it losslessly (well, you lose a lot, but only four channels, and you don't have surrond sound and a sub-woofer, do you?), you can download HeadAC3ch, and export it as w-channel lossless wav. Or just about anything else.