This has been posted before, but I'll add my explanation for those who might have trouble following the other answers.
To change the Message+ Notification Sound on a Galaxy Note 8:
1. Tap on the Message+ icon in your apps or on your home screen
2. On the top bar, to the left of the word "Conversations", tap on the three lines to bring down that menu.
3. Tap on the word "Customize" in that menu (next to the paint pallet icon) to open that menu.
4. Tap on the word "Tones" to open the assortment of sound choices
5. Tap on the various choices to hear them, ending with the one you decide to use.
6. Back out of the menus. Your tone in MESSAGE+ is set.
NOTE: IF you have chosen a different tone in the MESSAGES app, and you somehow later make MESSAGES your default texting app, you will no longer hear the MESSAGE+ tone you selected. Instead, you will hear the tone for MESSAGES
To change the Message+ Notification Sound on a Galaxy Note 8:
1. Tap on the Message+ icon in your apps or on your home screen
2. On the top bar, to the left of the word "Conversations", tap on the three lines to bring down that menu.
3. Tap on the word "Customize" in that menu (next to the paint pallet icon) to open that menu.
4. Tap on the word "Tones" to open the assortment of sound choices
5. Tap on the various choices to hear them, ending with the one you decide to use.
6. Back out of the menus. Your tone in MESSAGE+ is set.
NOTE: IF you have chosen a different tone in the MESSAGES app, and you somehow later make MESSAGES your default texting app, you will no longer hear the MESSAGE+ tone you selected. Instead, you will hear the tone for MESSAGES
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