Sound chips in laptops normally work properly or fail completely. I've never heard of one that fades over time. What about something stupid - has the speaker grill got bunged up with something?
The only other thing I can think of is that the small magnet at the back of the speaker has become detached or the speaker cone is damaged in some way. Trouble is that means taking it apart. Give us a make and model number and someone on the forum might be able to point you to some dismantling instructions.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.