I agree with the commenters that as a listener to the presentation you should probably avoid commenting on the lecturer's clarity (eg because of poor English) unless you are in very friendly and constructive terms with her.

However, I think you could still politely tell her that the presentation was interesting but you couldn't follow perfectly given the poor sound/background noise, and so you would like to receive a copy of the slideset. Then you could suggest to make the slides more self-explanatory.

Your question is more interesting however from the viewpoint of moderators or scientific committee member. In such a case, they have the explicit responsibility to the audience that lectures are clear and understandable. Otherwise, what would be the point of inviting people to lecture at all? Most importantly, a genuinely constructive feedback will be important for the lecturer himself, to avoid future embarassments.

A simple suggestion for anyone with language difficulties is to create/modify a slideset by adding as many self-explanatory phrases as possible. This creates redundancies for a competent lecturer, but saves a poor one. Eventually, such a slideset will only leave the room for poor pronunciation, but all key messages will be firmly delivered.