(128+127*sin(t/10))*((t>>10&3)+1)*(1+cos(t/10))

128 + 127 * sin(t / 1000): Generates a sine wave oscillating between 1 and 255. The frequency is scaled so that it fits within the 8 kHz sample rate. t / 1000 adjusts the frequency of the sine wave.
(t >> 10 & 3) + 1: Extracts a 2-bit pattern from t and adds 1 to make sure the result is always non-zero. This introduces variation based on the bitwise manipulation of t.
1 + cos(t / 5000): Adds a cosine modulation factor. The cos(t / 5000) oscillates between -1 and 1, so adding 1 scales it to range from 0 to 2. This modulates the amplitude of the waveform.