I would simply let your users fill in the comment textbox the way that they want to:
1) If they like using the mic button on their keyboard, then they will already be familiar in how to use that method, so there is no need to modify or reinvent/replicate it.
2) If the user likes to manual type their message, then let them. If they don't format the text, then you should still be able to easily understand their feedback.
But, if you really want to make it easy for your users to leave feedback and if they speak the same language as you, then you could simply allow them to record their voice (audio) and then your app will send the recording file to you and you would simply play it back - can't get any easier for them!