Yes, it's possible to effectively change the volume if you're using Pulse Width Modulation (PWM), although the timbre of the note is also affected depending on playback hardware and psycho-acoustics. [Dr. Blake Troise][1], who makes chiptunes under the moniker [Protodome][2], describes how in a recent paper:

[The 1-Bit Instrument: The Fundamentals of 1-Bit Synthesis, Their Implementational Implications, and Instrumental Possibilities][3]

> Even though the amplitude is a constant 1-bit waveform, the narrower
> pulses provide a way of varying volume. These narrower pulses have
> incrementally less power overall to the listener; as the duty cycle
> approaches 0% (or, by inversion, 100%) the perceptual volume decreases
> with it, even though the amplitude remains the same. This effect is
> not a consequence of the reduction of the pulsing signal's actual,
> electronic, or kinetic power. Instead, the reduction in volume is a
> product of bandlimiting—the effect whereby frequencies beyond a
> particular value are not heard. [...] Thinner pulses are constructed
> from more powerful high-frequency harmonics than lower ones.
> Accordingly, as the pulses get thinner, with extremely small or
> extremely large duty cycles, these higher frequencies increasingly
> fall outside the limits of what can be replicated by the speaker.
> Since these elements are not present, the result is a reduction of the
> waveform's overall power.

Some of the earlier uses of PWM on the platforms you mention were programs from around 1980 by [Paul Lutus][4] including Electric Duet. He states:

> Decreasing the duty cycle of the generating waveform increases the
> amplitude of high-frequency components while reducing the overall
> volume.

There were routines published in a few Apple II magazines that made use of this volume technique. Here's one in [Nibble magazine][5]:

> Software Volume Control	Goetz, Philip	November 1984

It's called VOLUMETONES.DEMO on [disk NIB22B.dsk][6].