Yes, it makes perfect sense to use one of the spare op-amps within the package for the audio amplifier section.V1 and V7 vary in sync, V1 sets the frequency of the output ramp wave by sending a reset pulse, the speed at which it is reset depends on the timer value sent by a microcontroller, (connectect to a midi device). subsequently, the V7 is set so that it charges the capacitor at a rate so that an amplitude of 5v is produced just before reset is sent to V7. Therefore the output amplitude Is at a constant 5V, even when the frequency varies. My design above already has 2 free TL084 on the IC, so if there is no downside to using them. Then I guess it would make sense to save space no? also, is the LM386 just a IC of the LIN topology you displayed? if so, would it not make sense to use this ?
On a practical note, it might be worth configuring the 4th unused op-amp within the package as a unity gain buffer (with no input signal) by shorting the output to the inverting input. Otherwise with two floating inputs, combined with its open loop gain could result in unstable operation due to stray voltages.