Hi @a51
Poor sound quality may be due to a failing battery. Even rechargeable batteries have a lifespan.
Try changing the battery and check if the sound is better.
Here's a video that may help. I realize that it is not about changing the battery but it shows how to open the headphones and then the battery can be accessed (see battery at 1:19 minutes into the video) and disconnected from the circuit board and replaced.
Search online for CS-JBE300SL battery to find suppliers that suit you best.
As to why the 3.5mm stereo audio cable connection to the laptop doesn't work it may be a faulty jack in the headphones or the jack may have come loose from the board. Have you tried using a different compatible cable just in case it is the cable that's faulty?
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