2 transistor oscillator

What part of the text are you reading that "says I should hear a changing in the oscillation"? The frequency of the oscillator is varied by changing the value of R1 or C1. Smaller values (within certain limits) will result in higher frequency of oscillation and larger values (again, within limits) will result in lower frequency of oscillation.

 

I would only expect a single tone. each transistor inverts the signal with gain so the capacitor feedback

 

This is a multivibrator, collector to base, collector to base. With two similar transistors, one will be on when the other is off. In this case, Q1 is npn and Q2 is pnp and both transistors go on and off together.

Assuming the oscillator is running, when Q2 is on, the collector is pulled high, when Q2 is off the collector is pulled low by the load. Thus the capacitor drives Q1 with a 9V square wave. The base of Q1 cannot rise much above 0V so the base will run from about +1V down to -8V. R1 pulls the base back up to start the progress over again. The frequency will depend on the time constant of R1 and C1.

Note that Q1 base can go down to -8V or so which is more than should be applied to the base. A diode in series with the Q1 base will protect it or you could run from a lower voltage, say 6V.