This is most likely because of liquid damage to the internal components. I haven't dealt with liquid damage before, so i recommend going to your local repair shop and ask them exactly whats wrong.
Has the speaker recently come in contact with any sort of dust/water (steam too)? Is the volume turned up? Do you have headphones connected? If the volume is up and there is no headphones connected, then it seems like you have a faulty speaker. I could be wrong though, have you tried resetting your phone?
It would be best to send it to someone who unlocks iCloud locked phones. The link you provided seems sorta reliable for it, but it would be best to check the website's reviews first before you buy anything. Or you can go to your local repair shop and ask them if they know how to unlock your phone or if they know someone who does. Good luck and i hope you get your phone fixed!
