I'm having the same problem. See my post just above yours here:
http://www.japanesepod101.com/forum/vie ... php?t=4380
So far no luck in getting it fixed. But I'm glad I'm not the only one with this issue.
Have you had any trouble in the dictionary or anywhere else? I've noticed sometimes the dictionary won't play back a sound but if I leave the page and go back (or re-search the word) it'll work.
Also, try changing the number of cards you want it to show you in your wordbank flashcards. For example, I've noticed that if I only select 25 cards I have no problems. But if I increase the number of cards the problems get worse.
Hopefully they can figure this issue out!