Where have the 8 bit sound chips gone?
As RedGrittyBrick says, they're not in production any more.

It's worth stepping back and asking why, and what's replaced them. There are three obvious candidates:


SID. This was synonymous with the C64, a product of the same company, and therefore died with MOS technology/Commodore. It was also a mixed-signal chip containing a number of nonlinear analog filters and artefacts that contribute to its 'unique' sound. Like valve amplifiers, it's not an accurate sound but a sound that people are fond of.
Yamaha OPL series (as used by Sega, Adlib etc). This was developed as a byproduct of Yamaha's instrument business. Pure digital, not mixed-signal. Yamaha have abandoned the FM synth approach and now use wavetable in their instruments.
Texas Instruments SN76489 (as used in the BBC micro). Again a digital direct synthesis chip that simply produced square waves. That functionality is trivial now on even a low-speed microcontroller.


Commercially, everyone is using either sample-based sound generation or digital direct synthesis from an algorithmic version of FM. Analog mixed-signal chip design is difficult, consumes a lot of chip area, and frequently requires expensive bugfixing. No sensible manufacturer is going to do one of those when there's no market.

Whereas doing it in software is cheap, accessible, and much easier to debug and tune. You might want to add external analog filters, if you're doing your own boards and don't care much about cost. Or there might be milage in trying to make one of the cheaper CPLDs emulate something like the OPL chipset. The Atari 2600's TIA should definitely be duplicatable, although the original design is incapable of producing notes that are in tune.