Audio output is provided by the Audio Processing Unit (APU), a Programmable Sound Generator (PSG) chip featuring four channels .
Interestingly, this is one of the few components that remained unchanged across Game Boy revisions. In fact, unlike the CPU, it cannot be accelerated: altering the speed of the oscillators does not improve sound quality, but simply changes the pitch. Instead, more circuitry would be needed to increase the capabilities, and that would also raise the cost.
Functionality
Let's take a look at the offerings. Each of the four channels is reserved for a single type of waveform:
Pulse
(Video) Pulse 1 channel.
(Video) Pulse 2 channel.
(Video) All audio channels.
Pulse waves have a very distinct beep sound that is mainly used for melody and sound effects.
The APU reserves two channels, each dedicated to a pulse wave. The channels offer four different tones (constructed by varying the pulse width). The first channel features an exclusive sweep control.
Due to the limited number of channels, melodies are often interrupted when sound effects are triggered during gameplay. This is especially noticeable in games like Pokémon Red/Blue when, during a battle, the Pokémon's cry takes over all music channels.
Wave
(Video) Wave channel.
(Video) All audio channels.
The third channel of the APU allows to define a custom waveform. The wave is composed of 32 4-bit samples which are stored in a wavetable.
This channel also allows control over its volume and frequency; the latter enables to produce different musical notes from the same entry.
Noise
(Video) Noise channel.
(Video) All audio channels.
Noise is basically a collection of random waveforms that sound like white static. One channel is allocated for it.
It is commonly used in games for percussion or ambient effects.
This channel offers only two tones, one produces clean static and the other generates robotic static. Its frequency can also be adjusted.
Secrets and limitations
The mixer outputs stereo sound, meaning channels can be assigned to the left or right side. In other words, the audio subsystem offers panning controls. This is only audible through the headphone line, I'm afraid, as the built-in speaker is mono.
Furthermore, the mixer hardware is also connected to a dedicated pin on the cartridge, allowing the latter to stream an extra channel, provided the cartridge outputs the analogue sound (which is only possible with extra hardware). Nevertheless, no game in the market ended up using this feature, leading to its removal by the time the Game Boy Advance (and its backwards compatible subsystem) arrived.