Have you tried playback speed on youtube? Earlier this feature was launched for youtube web. Now, youtube has rolled out this feature for Android and iOS.
In an announcement post a Software engineer Pallavi Powale wrote — “To speed up or slow down audio while retaining its comprehensibility, our main challenge was to efficiently change the duration of the audio signal without affecting the pitch or introducing distortion. This process is called time stretching. Without time stretching, an audio signal that was originally at 100 Hz becomes 200 Hz at double speed causing that chipmunk effect. Similarly, slowing down the speed will lower the pitch. Time stretching can be achieved using a phase vocoder, which transforms the signal into its frequency domain representation to make phase adjustments before producing a lengthened or shortened version. Time stretching can also be done in the time domain by carefully selecting windows from the original signal to be assembled into the new one. On Android, we used the Sonic library for our audio manipulation in ExoPlayer. Sonic uses PICOLA, a time domain based algorithm. On iOS, AVplayer has a built in playback rate feature with configurable time stretching. Here, we have chosen to use the spectral (frequency domain) algorithm.”