It seems that it's not yet possible to give audio or video feedback for an assignment. However, the feature is being developed and will hopefully be available in 3.6. Please feel free to watch or vote for MDL-27520.
Thank you for your reply!
I was kind of surprised that this is not possible with RecordRTC because we used Poodll up to this point. Poodll does allow for audio and video feedback on assignments. When RecordRTC was deployed in Moodle 3.5, we stopped using Poodll and then found out is was not possible to do this in RecordRTC
Let's hope for 3.6 then. We can't wait, teachers and students were getting used to video/audio feedback. Much more personal!