Well, not only doesn't the coin sound like silver does, also the weight isn't right.
It weighs only 22 grams, where it should be 25.
The surfaces aren't smooth like silver, but slightly bumpy and even small dots are found.
The differences in design are also very clear. You can see some in the following pictures.

The fine lines are very crude and under the shell, there is even an extra vertical line there, where it shouldn't be at all. When you have a real one to compare with, the differences are very clear.
If you are confronted with a doubtfull one, always check the weight first. That's always a good reference, not conclusive, but many forgeries were only in the region of the weight of the real ones. Contemporary forgeries were only made to fool the common man, so weight wasn't the main issue. The appearance was the most important part of the task at hand.
Hope this is clear enough. If not, please say so

Regards,
Jos