"I said it wrong."
I think that's probably OK. It seems a little choppy to me though (separating 说 and 错 seens choppy to me.) 错 is a verb compliment here do it should comr right after the verb.
Yes. Otherwise would mean something like "I speak wrong". The 了 (le) marks that is a completed action (I said)
Yes, to indicate that the action is finished (expressed by the use of past tense in English).
The audio is incorrect. The "了“ should be pronounced in this case as "le" and not "liao" although "了“ is pronounced as "liao" in expressions such as "吃不了“.