is laughing of women in front of non mehram men okay?

I have to ask whether a female can laugh in presence of non mehram men. I used to think that it's not correct as females must use stern or strict voice with men as is mentioned in Quran 33:32. In this verse, laughing is not mentioned but it seemed obvious to me that laughing is going against being having stern voice. But recently while discussing this with someone, I realised that I may be wrong. He said that talking in loving manner is wrong but laughing is not wrong and it's not non-stern thing according to him, specially when the conversation is related to work only [casualness pops up even in work related talks]. So what's correct and how is it derived? And what is its level of importance like farz, wajib, recommended or optional? Jazakallah.