Hello P4nthaa.
Thanks for reaching out to Apple Support Communities. We understand that the volume from your Bluetooth speaker is still able to be heard though the volume control is at the lowest setting. We will be glad to assist.
First, make sure that you have the latest iPhone update:
Update your iPhone, iPad, or iPod touch
Additionally, "If this is the first time that you tried to use the accessory, make sure that it works with your device. Some accessories only work with certain models of iPhone, iPad, or iPod touch. If you arenât sure, contact the manufacturer."
If you see an âAccessory may not be supportedâ alert
Next, Forget and re-pair the speaker to see if that helps.
"To unpair a Bluetooth accessory, tap Settings > Bluetooth, find the device you want to unpair, and tap the info button  , then Forget this Device. This will remove the accessory from the list of available Bluetooth devices.
To add the Bluetooth accessory to your device again, place it back in discovery mode and repeat the steps above to pair it."
Pair a third-party Bluetooth accessory with your iPhone, iPad, or iPod touch
Hope this helps.
Cheers.