Port, starboard, midships and aft are mentioned, being mentioned means they are there. Where exactly is the question for those phasers. You, I believe, brought up they were mentioned in dialogue.
TAS would seem to be canon, or at least reference material; or "Questionable Canon" as Memory Alpha puts it in the links previously provided.
However, In A Mirror, Darkly only showed the ventral phasers and the aft phasers providing dorsal shots. The Defiant in that episode even tilts to one side to fire torpedoes at the ship attacking her dorsal side. If there was ever a time for dorsal phasers on the saucer to fire, that would have been the time.
So yes, where are the emitters?
After taking a look at "In A Mirror, Darkly" I think you're going to have a different answer depending on which TOS continuity you're looking at.
- "Balance of Terror": Forward, Port, Starboard and Midship (actual locations unknown)
- "Arena": Aft (actual locations unknown)
- Most episodes: Majority beams firing from the forward ventral saucer area.
- "Balance of Terror", "Errand of Mercy": Pulses firing from forward and starboard ventral saucer area
- "The Doomsday Machine": phaser firing from "port side" of the ventral saucer dome.
- Phasers in dialogue (same as TOS)
- Phasers seen (standardized to the same forward ventral saucer section on the lower dome except for...)
- "The Doomsday Machine": two saucer forward dorsal phasers fire at DDM from Enterprise
- Phasers in dialogue
- "In A Mirror Darkly" - forward emitters
- Phasers seen
- "In A Mirror Darkly"
- aft phaser emitters visible and fired
- two forward ventral saucer phaser emitters visible and fired. they later are no longer visible in later battle. they are positioned higher than on the TOS-Remastered version.
- no visible dorsal emitters on saucer
TAS - the dorsal firing as mentioned by Timo.
Regarding "In A Mirror Darkly" - the aft phasers firing direction pretty much tells us where the enemy ship is so when it is pitching up to fire torpedoes the torpedoes are still having to hook a turn towards the target from what I can tell. The lack of dorsal saucer emitters suggest no phasers there, but curiously the forward ventral emitters that were present in the dock are no longer visible in that same battle. Did the emitters hide themselves once the ship was operational again? If yes, then that could suggest dorsal saucer emitters that were hidden and the aft phasers might have been used because they had a better LOS, IMHO. If no, then there are no dorsal saucer emitters in the Enterprise-series USS Defiant.
But, there are dorsal saucer emitters in TOS-Remastered.