May just be a big or something, but I noticed in Firefight matchmaking, the Firefight voices do not work when you throw a grenade, but they do in custom Firefight. For example, my Firefight voice is Johnson. When he throws a grenade, he'll say things like "Frag out!" Or "This is for Harvest!" Not necessarily game breaking, but some of that dialogue is pretty humorous. Not to mention it's part of the game, therefore should be fixed.
May just be a big or something, but I noticed in Firefight matchmaking, the Firefight voices do not work when you throw a grenade, but they do in custom Firefight. For example, my Firefight voice is Johnson. When he throws a grenade, he'll say things like "Frag out!" Or "This is for Harvest!" Not necessarily game breaking, but some of that dialogue is pretty humorous. Not to mention it's part of the game, therefore should be fixed.
It may be more helpful to submit a bug report ticket at

The Halo Support SiteThere is an article explaining how to submit a ticket but I had some trouble finding it initially. It's under MCC known issues at the bottom of that article.