What is a better way to say the phrase 腳都軟掉?
I suggest 「翻倒」and  that is "flipped" in English

Example:

「他此言一出，眾人翻倒」; "Once  he said that, everyone flipped"

Also, in this kind of scene, the audience was figuratively "being knocked out" or "floored". I found the idiom "blow someone away" be a good match, and from the example sentence, I found "knock someone off their feet" suit even better.


  http://idioms.thefreedictionary.com/blow+away
  
  Blow somebody away also blow away somebody:
  
  
  To cause someone great pleasure or surprise knock somebody off their feet 
  
  
  "Everyone I tell that story to, I mean everyone, is just blown away." 
  
  Related vocabulary: blow your mind


Example:

"Once he said that. Everyone was blown away!"

"His stupid words knock everybody off their feet!"

"His stupid words floored everyone!"

As for Chinese slang, 「雷倒」http://baike.baidu.com/view/1816380.htm describe the reaction scene  very well.

Example:
「他的蠢話，雷倒眾人」