2017-03-27 (updated 2017-03-31)

The Altair could be "upgraded" with an 8086 or z80 CPU.

The Altair 680 models are a different architecture for 6800 CPUs. Not compatible with the 8800.

The Altair had no sound hardware at all for most of it's popular life. Not even a speaker, not an audio out. NO SOUND. But you could make sound using an Altair anyhow.
Note this video also demonstrates actual 3-voice sound hardware made for the S-100 bus years after the Altair was released. Later, TRS-80 devs would use the AM radio method to bring sound to TRS-80 model Is.