Not necessarily.
He might have been carrying his ipod with recorder attachment and punched record. Wouldn't be that hard to do with two yelling people fully absorbed in their own thing.
D
Setup or no, that recording apparently was enough to convince the powers that be not to charge her.
Might have been a replay of other fights, and she or they decided to get it on tape, for a divorce action - and, thinking it over, added the .38 - just in case.