The Commodore 64 did not have any hardware dedicated to digital audio playback, so how were some games and songs able to achieve this on an unmodified and unextended Commodore 64? Did these techniques require specific hardware revisions?
No hardware revisions are necessary. The digitization playback is achieved by bit-banging the volume register of the SID chip to simulate a digital playback device. Essentially, the SID can be used as a 4-bit digital playback device. What's amazing is that the thing sounds as good as it does playing back digital data.
A great article about digis on the C64 can be found here: http://sid.kubarth.com/articles/the_c64_digi.txt
The best audio quality was obtained with
pulse width modulation using the square wave voice. The oscillator was set to 0 and the pulse width was set by flipping the pulse width from 0 to maximum.
With the CPU speed at 1mHz the fastest pulse switch rate achievable is around 15kHz, producing a sound wave of 7.5 kHz or less. Setting the low pass filter help smooth the sound.
There were also some more recent creative alternatives taking advantage of filters and undocumented test bits as well. Check out the Cubase64 demo by Mahoney: http://www.livet.se/mahoney/cubase64 You can also find versions of it playing on the real thing on YouTube, naturally. (spoiler alert: your face will melt -- it's that awesome.)