Speeding up or slowing down music alters the acoustics of the space the music is performed in. If you slow down music, for example by halving the playback speed on a tape recorder, reverberation times related to the size of the room will double, and the room will seem to have increased in size.
Duration of a composition will also double, and the pitch of all instruments and voices will sound one octave lower. Women's voices will be magically transformed into men's voices.
The timbre of the recorded sounds will also change. This happens because timbre is largely determined by the relative position and volume of the harmonics, and when you halve the speed of a recording all harmonics will drop one octave.
A strange effect of slowing down recorded speech is that the speaker seems to be unable to open his mouth wide enough to articulate properly. He seems to mumble, and is hard to understand.
Slow down speech even further, and it is turned into a slow, threatening, dark, creepy slur from the Netherworld, uttered from a mouth that practically seem unable to open at all!
For me, discovering all this stuff was incredibly exciting. It led to a wealth of new musical possibilities that I investigated extensively between 1979 and 1984, and the best results found their way onto various Kubus cassettes.
Metaal (Metal) and Vertraag-de tijd (which means Slow down time as well as Time slowed-down) resulted from time delay experiments; De lege aarde (The empty earth) on Leven na de bollen (Life after the spheres) and Digit 2 from experiments with harmonics.