There is no sound out from the 34 pin cartridge port, though nothing prevents a cartridge from packing an extra memory-mapped device like a sound chip.
The side port has an AUDIO IN though. Speech and MBX are the only peripherals to use it AFAIK.
There was Marc Hull's Sid Master, a card for the Pbox. (last produced 2005 ish?)
There have been MIDI cards.
FORTI was a 1982 project by a TI engineer (Lee Cantwell?) which put 4 TI sound chips on one card.
I am working on FORTI-2 which will be a sidecar peripheral and offer unlimited stereo audio possibilities.
Like the original FORTI, its native language will be FORTH, but it will be accessible from any language, including access to 12 voices from BASIC.
You will be able to load waveforms from BASIC with the delightfully wacky format CALL SNDCHAR8(1, "01FF") (set voice 1 to a simple 8 bit sawtooth wave, interpolated.) To put it back to a basic square wave, CALL SNDCHAR8(1, "01FF",
I am testing with the free AKWF set downsampled to reasonable memory size for the 4A (they sound.. not as good).
As a bonus, AKWF has samples for a lot of 80s sound chips!
FORTI-2 also has its own SD card slot, so loading the samples by name should be a much better way to do it. Imagine CALL WAVE(1, "AKWF_0167.WAV")
The changes persist (FORTI-2 requires external power) so you can change the timbres and then play Parsec, for instance. (I'm using a sound list from Parsec in my testing.)
I have over 200 hours into this project in 2018. Mostly spent debugging (current bug has FORTI-2 occasionally disrupting a memory cycle, typically crashing the 4A.)
Edited by FarmerPotato, Mon Jan 7, 2019 4:23 PM.