If you are experiencing poor sound quality when using soundcore Sleep A20 in Bluetooth mode, there are a few things you can try to improve the audio performance:

1. Make sure you wear the ear tips and ear wings correctly. Choose the ear tips and ear wings that fit your ears the best, insert the earbuds into your ear, and find a suitable position for the best fit. Note that changing the sealability of the earbuds may improve the sound quality to some extent

2. Try setting a different EQ via the soundcore app and customize the EQ to your liking.

3. Reset the earbuds as follows:

- Place the earbuds in the charging case and leave the case open. Press and hold the button on the charging case for 10 seconds until the LED indicators on the case flash 3 times. This confirms that the reset was successful.

4. Try using a different sound source or music app. Make sure that the audio source you are listening to is free of distortion or noise. 

5. Use a Q-tip with a bit of rubbing alcohol to gently clean the metal mesh filter under the rubber ear tips.