Try the following:
Turn off and choose "Flat" for any bass, treble or other EQ and sound processing settings on your Bluetooth® device. Check for these settings both in the device's audio settings and also in the settings for the app that is being used
Set the volume level of your Bluetooth® device to 85%. If the volume output of the device is too loud, it might cause distortion in connected speakers. If the volume of the device is too low, it can also affect the sound
Try a different app. If sound quality is OK in one app but not in another, the app is the issue
Try moving your device closer to the speaker. Bluetooth® range is about 9 metres. Range is reduced when interference is present.
Interference includes things like walls, multiple Bluetooth® devices in the same area, pockets/purses/bags, wireless routers, microwaves, cordless phones, etc.
Reduce the number of applications running on your Bluetooth® device. Running multiple apps can reduce reliability if the device is trying to do too much at once. Close non-essential apps and check if the sound improves
Try turning off Wi-Fi® on the device
Go to Bluetooth® settings on your device, select the SoundLink® speaker and choose to "Forget" or "Remove" the speaker. Next, briefly hold the Bluetooth® button on the speaker until the Bluetooth® light begins flashing blue. Then try again to pair the device to the speaker
Turn the speaker off and back on
Disable Bluetooth® on other connected devices that are not currently in use. The speaker will pair to the two most recently used devices in its memory. Sound might drop out if a second connected Bluetooth® device goes out of range of the speaker
Reset the device you are trying to pair. This is typically done by rebooting/restarting it or removing the battery and/or power cable for one minute
Reset the speaker by turning it on and then holding the power button for 10 seconds until it turns off.
A factory reset clears the speaker's language setting and its paired mobile device memory, returning the speaker to original factory settings. Once reset, turn the speaker on and try again to pair the device
Try pairing another device. If the other device works, there may be an issue with the first device