I'm curious as how you achieved this. I'm assuming that you're working with audio files and not physical media if you downloaded it. While I know it can be done digitally such as in transcription software or audio editing software suites, but I haven't seen that feature in general media playback software that I've used.
Stepping down a record player speed or open reel tape recorder wouldn't achieve the effect properly. Furthermore, it's not only Dave's voice, but the music too, that would be slowed down to sound very slow if the chipmunks were to fall into proper speed if it were possible to get the chipmunks sounding normal.
In older studio machines to get that rate of change, that effect had to be done by taking control over the primary capstan motor with a variable frequency oscillator amplified to working voltage for an ac synchronous motor or latter DC controlled phase locked pulse width modulated motors to achieve a proper voice color. That wasn't a common feature found in audio recording studios unless it was audio for film and the transport had to be locked up and resolved to another source.
Chipmunk voices would be a relatively smaller change in speed then the standard geometric progression of speeds available on open reels. IIRC, a 100% change in linear spead represented an octave change. Even the smaller change between 45 rpm and 33rpm on a turntable may have been too much.
I try to find new songs for my Christmas library online, and this year discovered a bunch of standards recorded by the tribute band 'The Fab 4', all worked around Beatles songs. Very cleverly crafted, and when I had them on in the background, it took my guests a little time to register what was going on.
The Chipmunks' "Christmas Don't Be Late" is hilarious slowed down where the trio's voices are normal speed and Dave sounds demonic (and I'm sure every kid back in the day found this out).
Probably my favorite Christmas album is Leroy Anderson's - starting with 'Sleigh Ride' of course, followed by a full orchestral 'Christmas Festival', then three suites of carols - for brass choir, string orchestra, and woodwind ensemble. This one gets played through the Twelfth Day of Christmas.