As far as I am aware, celluloid/mpv have adapted the "WSOLA" (Waveform-Similarity-Overlap-Add) algorithm for speeding up audio, the same that the Chromium browser uses since 2018.

However, the audio is muted above 4× playback rate.

While speech is inaudible at that high speed, such high speeds are still useful for fast seeking. The current web browsers mute audio above 8× playback rate. As of 2017, Firefox actually did mute audio starting at 4×, but they have thankfully increased it.

Please don't mute the audio at 4× speed. If a web browser can play audio at 8× speed, a dedicated media player should too. And ideally even more than that. As fast as the computer is capable of handling.