Is it possible to use a 558 quad-timer IC as a polyphonic synthesizer?
The most common way of controlling frequency with the 555, 7555, and other chips in those families is by adjusting the control voltage.  On the 556, the two timer circuits each have a separate control voltage input, allowing their frequencies to be controlled independently.  On the 558, however, there is one control-voltage input which acts upon all four of the timing circuits simultaneously.

It may be possible to use the 558 as a four-voice synthesizer if one were to drive the oscillators with four separate programmable current sources.  The outputs would likely just be short pulses, however, unless one added some more complex circuitry which would reduce the linearity of the current-to-frequency conversion.  While using a 558 as a four-voice synthesizer in that scenario would certainly be possible, the circuitry required to support its use in such a role would likely exceed what would be required to build a synthesizer via other means.