The issue is described in the Apple support discussion in the below link and it seems there are a number of users reporting the same issue.
I just bought a used 2017 MBP15 with a larger SSD because my other 2017 MBP 15 was running out of space and found the one I just bought has this muffed audio issue. My original MBP does not have any audio issues but the other one I bought will have muffled audio after a Youtube video is played more than few seconds - which can be temporarily reset by running ‘Audio MIDI Setup’ and change the audio frequency (Hz) of the Internal Speakers output.
https://discussions.apple.com/thread/8014046?page=1
I just bought a used 2017 MBP15 with a larger SSD because my other 2017 MBP 15 was running out of space and found the one I just bought has this muffed audio issue. My original MBP does not have any audio issues but the other one I bought will have muffled audio after a Youtube video is played more than few seconds - which can be temporarily reset by running ‘Audio MIDI Setup’ and change the audio frequency (Hz) of the Internal Speakers output.
https://discussions.apple.com/thread/8014046?page=1