[petelanger]

Yes this is normal. To vary the speed and maintain the pitch, the encoder/decoder (CODEC) has to replay sampled audio multiple times to slow down the speed or remove chunks of audio to increase it. Very high quality CODECs can do this a bit better, but there is always going to be distortion (possibly a warble or choppy sound) because you’re not playing the original audio anymore. Analog recorders like tape would give you truer sound, but the pitch would be heavily altered.