Most of (but not all) chip sounds are synthesised by simply dividing a clock square wave to get a square wave of desired frequency, and sometimes using a sawtooth/triangle wave from volume LFO or an (ADSR) envelope to get some kind of ring modulation. LFOs are used to control or influence a sound parameter such as pitch or filters in a repeating cycle.
More recent "old school" or "demostyle" MOD music, although sample-based, continues the style of the chiptunes used in these intros; new compositions in this style can still be regularly found at www.chiptune.com or www.chip-on.com (new chiptunes from old computers/formats can be found here as well).
