The Thunderator sound was never updated to the new duration. It still lasts 4 seconds, which is enough for 3 strikes. But the spell does 4 strikes now. Also, the +4 strikes talent makes it last a total of 14 seconds, but the thunderator sound still lasts 4 seconds.

Best solution would be to make it a looping sound.

Here's a looping version of the sound (download)

Here's a comparison between the current and the looping version