The choice “fixada” refers to the voice message that had been pinned, while “fixou” refers to the person who has pinned the voice message. I believe that pinning a message on your device is a way to keep your most important messages on the “spotlight” by adding it to o priority spot at the top of your other messages so it is always easy and fast to access; therefore the focus is the action/ message itself not the person who has practiced it and for that reason “pinned” should be considered in the past participle, not simple past.