OMG, Gnay! I'm very sorry to hear about this. Have you tried checking the setting? If not, you may try to do this: Settings > Sound and Vibration > Notification > Sound
I hope this works! If not, don't hesitate to reach us via our TOLL-FREE mobile number #GALAXY (#425299) or via our TOLL-FREE LANDLINE: PLDT Toll free : 1-800-10-726-7864 Globe Toll Free : 1-800-8-726-7864 Standard Landline : (02)8-422-2111 Daily (8:00 AM to 8:00 PM) or via CHAT SUPPORT: https://livechat.support.samsung.com/Customer_new/PH
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