You should understand basic quest/scene design.
The marker should have Stores Text checked. Go to Miscellaneous>Message, right click and create a NEW message. Set your quest as the owning quest, make it a messagebox, give it an ID, and make the Title your NPC's name. This NPC is the one who should be perceived as speaking when the xMarker is actually talking. With the Meridia's Beacon example, the Title would be Meridia.
xMarker: A new hand touches the beacon...
Add a new actor to your scene, and make it the xMarkerVoice alias, NOT THE ALIAS OF THE NPC. Add a new phase. I'm just going to make one phase for the scene - in it the xMarker will say about 7 lines (7 responses in 1 info). However, you can make two phases and have the first line say when started, and then only say the next ones when you pick up an item by adding start conditions to the second phase. You will not have any packages on your xMarker, because xMarker's can't move.
So, I add a new action to my first phase, and make it Dialogue. I make a new info within the dialogue - I don't need to set emotion or headtracking, because you can't see the XMarker. In the info, I add as many responses as needed for my marker to say everything. For every new line in a scene, you should make a new phase with a new action. Having it all in several responses works, but can cause issues where the scene won't interrupt if it needs to be interrupted. I don't add any conditions to my phase(s), since I'll be starting it through script, and since the actor speaking is the xMarker it determines only it can say it. You CAN add conditions to the dialogue if you wish. Once you're done setting up your line, set the Audio Output Override to SOMDialogue2D. This is optional, it will just ensure the dialogue can be heard from far away. Absolutely essential - you must set the Speaker of the line (upper right of the info window) to the ID of your actor who should be seen as speaking. This will not let you use an alias for the speaker, which is unfortunate but required. You'd have to make several different infos in the action w/ different speakers and conditions checking who the alias is if the xMarkerVoice could be different people. I also checked Post Processing Required, but it is not necessary. I just did it because the Meridia's Beacon quest did. If your NPC is supposed to be talking normally, it's probably not needed.
Also, however you start your scene, you must move the xMarkerVoice to the player. If you start it when the quest starts, make/use a Start Up stage, and in it, do:
Scene Property xMarkerTalkingScene Auto
ReferenceAlias Property xMarkerVoice Auto
; fill your properties:http://skyrimmw.weebly.com/skyrim-modding/how-to-fill-properties-skyrim-;modding-tutorial
Event OnTriggerEnter(Actor akActionRef)
If your quest is start game enabled, be sure you generate an SEQ, and if there is no voice yet - install Fuz Ro D-oh.
Note: You could have a scene between two xMarkerVoices (separate aliases) if you want. You could have a scene between an xMarkerVoice and a NPC, etc.
If you find any issues or found this helpful, please comment below.