Because audio for 23.976fps
film is playing at the same speed as 29.97fps
video. Which is really interlaced video at 59.94
fields per second. 3:2 pulldown is used to synthesize extra video frames to transfer to NTSC video so for a given instance of time, for every 4 frames of
film you get 5 frames of
video. (23.976/29.97 = 4/5) The instance of time remains unchanged so the audio is playing back at the same speed. This is all inherited from the CRT days before progressive video playback on flatscreens, and the video fps was tied to the local electrical frequency (50 Hz in Europe vs 60 Hz in USA / Japan) which is how the CRT timings were derived.
Coincidentally PAL video plays back at 25fps, to convert film to PAL they just speed it up from 24 to 25fps, audio and all, which is why if you watch a Euro DVD release of the same film all the actors sound like they just ingested helium before the scene because the audio is playing back ~4% faster. This was largely fixed with Blu-Ray as films play back at their native rate[1] on progressive displays but our European friends who have only seen films on VHS/DVD or TV and not in the theatre or Blu-Ray have been done a disservice, because the video and audio is sped up on playback and not as the director intended it to be shown.
[1] A notable exception is some unscrupulous distributors abroad release shitty PAL 25fps Blu-Rays of films which are simply upscaled PAL DVD versions that they then market as Blu-Ray but are worse than DVD quality but at marked up prices. This is more common than you think. Official releases from the studios are encoded at 23.976fps on the disc. TV shows shot on PAL video (native 25fps) will always play back at 25fps.
Coincidentally PAL video plays back at 25fps, to convert film to PAL they just speed it up from 24 to 25fps, audio and all, which is why if you watch a Euro DVD release of the same film all the actors sound like they just ingested helium before the scene because the audio is playing back ~4% faster. This was largely fixed with Blu-Ray as films play back at their native rate[1] on progressive displays but our European friends who have only seen films on VHS/DVD or TV and not in the theatre or Blu-Ray have been done a disservice, because the video and audio is sped up on playback and not as the director intended it to be shown.
[1] A notable exception is some unscrupulous distributors abroad release shitty PAL 25fps Blu-Rays of films which are simply upscaled PAL DVD versions that they then market as Blu-Ray but are worse than DVD quality but at marked up prices. This is more common than you think. Official releases from the studios are encoded at 23.976fps on the disc. TV shows shot on PAL video (native 25fps) will always play back at 25fps.