Just from the back of my memory, since I haven't done any YM2149 stuff in almost 15 years:
The envelope generator is used to create bass sounds in a variety of ways. By setting the frequency high in the audible range, the various ramping triangular and saw-tooth waveforms can be used as an alternative to the standard squarewave sound. Also, you can combine the pure envelope-generator sound with the squarewave from the oscillator to create a buzzing effect. Usually you use a frequency ratio that has both generators play either the same note, an octave up or down, or a fifth. To add modulation to the sound you can slightly detune the two oscillators.
The so-called SID effect requires use of a programmable timer interrupt, e.g. like the MFP Timers A,B,C, or D on the Atari ST. Here we use the native squarewave oscillator and additionally program the timer to trigger an interrupt at close to the same frequency of the oscillator. The interrupt service routine then toggles the volume register between zero and the current volume (whatever the volume value in the music player's ADSR envelope). The slight offset between the frequencies causes an effective pulse-width modulation of the square wave, which gives the pleasant movement in the harmonics known as the SID sound.
Tao also invented a technique called Sync-buzzer, which combines the use of interrupt and envelope generator techniques to create some amazing sounds reminiscent of ring-modulation or filtering. But I cannot speak about how that's done exactly, since I have never used it myself.
Hope this helps