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].

Probably the ultimate evolution of Apple II 1-bit audio are projects written using [Michael Mahon's][7] DAC522, including [RT.SYNTH][8] and [Digital Music Synthesizer & Drummer][9]:

> DAC522 is a software digital-to-analog converter for the Apple II that
> plays a stream of 11.025kHz sound samples through the 1-bit Apple
> speaker port using a pulse-width modulated (PWM) stream at a pulse
> rate of 22.05kHz, or two pulses per sample. The 22kHz pulse rate
> renders the pulses themselves virtually inaudible to human ears, but
> the average output, changed by varying the pulse width in proportion
> to sample values, reproduces the sampled sound to a precision of 5
> bits.

Regarding PWM and pitch, although RT.SYNTH is single voice, its instruments are resampled dynamically to any frequency and shaped with an envelope. Here's an overview:

> The fundamental problem a music synthesizer must address is the
> production of notes of many frequencies and arbitrary durations having
> specified waveshapes (voices). Storing all the needed combinations in
> limited memory is not practical.
> A workable solution is to store each waveshape needed as a
> single-frequency sample, then resample this waveshape on the fly to
> create any desired frequency.
> Most instrument sounds change as a note sounds. For example, many
> sounds have an "attack" that sounds different from the rest of the
> note. And many instrument sounds change in amplitude as a note is
> held, usually decaying in amplitude or changing in "timbre" or
> spectral composition. Synthesis of notes with changes appropriate to
> particular instruments, therefore, requires that the synthesized
> waveform change as a function of the length of time the note is
> played.
> RT.SYNTH performs all the calculations required to carry out these
> tasks while it is generating the pulses corresponding to the
> previously calculated sample.