Here's an actual chat transcript from Wednesday, March 29th, 2017 of my asking this exact question directly on Samsung's official tech support website itself (I was overly explicit in my inquiry to be certain before pre-ordering):
"[Me]: Hi, I couldn't find the model number selection for this chat, but I had a question about the just announced Galaxy S8/S8+ phone up for pre-order tonight at midnight. There seem to be conflicting reports on whether this phone has only one speaker for mono sound, or 2 speakers for stereo sound (reference one example article mention:
https://www.gizmodo.com.au/2017/03/the-galaxy-s8-is-samsungs-biggest-and-safest-bet-yet/ ) As I was considering pre-ordering this at midnight tonight, and my decision hinges on which specs are the correct ones, will you clarify this for me? thx

Dinesh N: Hello [me].
[Me]: Hello, Dinesh
Dinesh N: I do understand that you want to know about the speaker for the upcoming S8/S8+ phone.
Dinesh N: Am I right?
[Me]: Yes, specifically, whether the reports I've been reading that the earpiece speaker at the top of the phone also doubles as the 2nd loud speaker combined with the lower loud speaker to create stereo sound. Again, the article I referenced above is one example citing this as true, but then I've read other articles saying there is only one speaker for mono sound. Here's the article example (one of many) again for your reference:
https://www.gizmodo.com.au/2017/03/the-galaxy-s8-is-samsungs-biggest-and-safest-bet-yet/
Dinesh N: Thank you for the information.
Dinesh N: Not to worry, I see how important this is to you, I will be glad to assist you with the required information.
[Me]: Thanks
Dinesh N: You are welcome.
Dinesh N: Could you give me a few minutes while I will go through my resources and get information for you?
[Me]: Sure, that's fine.
Dinesh N: Thank you for understanding.
[Me]: No problem
Dinesh N: I have checked the information for you, it comes with a double speakers for dual sound.
[Me]: Cool, and just to clarify, that's dual speakers for stereo sound built into the phone, as opposed to the additionally provided connectable stereo headset, correct?
Dinesh N: Yes, you are absolutely correct.
[Me]: Perfect, thank you so much for your help
Dinesh N: If you feel I have done whatever I could in assisting you, you can give me feedback by clicking the round "Blue X" Mark on the top right in the chat window your chatting. Your feedback is much appreciated.
Dinesh N: Is there anything else that I can assist you with?
[Me]: No, that should do it, thanks again.
Dinesh N: Your feedback is much appreciated.
Dinesh N: If you feel I have done whatever I could in assisting you, you can give me feedback by clicking the round "Blue X" Mark on the top right in the chat window your chatting.
Dinesh N: Thank you for contacting Samsung Technical Support, we appreciate your business. If you ever need to contact us in the future, please save and reference this chat transcript ID 4346929342. Please provide us with your feedback, so we may serve you better by clicking on the blue “X” button to receive the transcript of your chat and fill out a brief survey. Have a great day!
info: Chat session has been terminated by the Samsung Agent."
(I gave him a positive rating.)