Why don't you ask her why she muted herself? You won't know until you ask her and then you can determine if she is telling the truth. Don't place more significance on it until you ask her about it.
My girlfriend had to talk to her ex over the phone and me and her were on skype and she muted her voice, what do you guys think of this? Is this a red flag she might be cheating?
Maybe she thought a conversation between her and another persdon was it was none of your bisiness.
You need to know if he or she really cheats so you can be certain about your thoughts. You contact Smith via his gmail account( Williamsdhackghost at gmail..com) I'm sure he'd help