Comment 22 for bug 1654480

I believe this is due to Firefox playing ~70ms samples of audio at original speed, and skipping between them. In my experience with a similar effect in mplayer (https://mplayerhq.hu/), blending samples of about 15ms produces quality comparable to Webkit (Chrome, Opera).

To test this yourself in Firefox and mplayer:

video: https://www.youtube.com/watch?v=K0Tsa3smr1w
["DASH Audio" stream URL](https://r1---sn-n4v7sn7y.googlevideo.com/videoplayback?id=o-AKrDYrKDiSnXJpeZyZF5pIoHpfZrgIKmRSc2E2753l3m&itag=250&source=youtube&requiressl=yes&mm=31%2C29&mn=sn-n4v7sn7y%2Csn-n4v7knlk&ms=au%2Crdu&mv=m&pl=18&ei=4vTNXNDJB4WQkwag0KugCg&initcwndbps=2038750&mime=audio%2Fwebm&gir=yes&clen=15794557&dur=2131.221&lmt=1507964730717576&mt=1557001371&fvip=1&keepalive=yes&c=WEB&ip=205.154.192.198&ipbits=0&expire=1557023042&sparams=ip%2Cipbits%2Cexpire%2Cid%2Citag%2Csource%2Crequiressl%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Cei%2Cinitcwndbps%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&signature=4BB77D089E75903DF0EB3B66A4C36139C0BA2B61.92C7BDB33502808BADBCD64236905EBB1B8CBEDF&key=yt8&ratebypass=yes) for same video (retrieved with youtube-dl -j K0Tsa3smr1w)

`mplayer -speed 2 -af scaletempo=stride=70 <media_file_or_url> # present Firefox quality`

`mplayer -speed 2 -af scaletempo=stride=15:overlap=1 <media_file_or_url> # better quality`
Note that using `]` to speed up greater than 3x, the words are still intelligible in the latter method. BTW: "overlap=1" causes the samples to be blended together across their entire duration.