This might be the toughest question(s) ever asked here. I like it, but it requires a lot of thought. Worth noting is that two identical models can sound different from each other if one has a rock-hard platen and the other a more absorbing one, or even if the action in one is well tuned and performing at its peak while another is dirty and has less than optimal hammer velocity. There are probably other factors as well, such as the sound deadening material having been removed from machine but not the other. Even a loose ribbon cover or case missing a screw or two can impact the sound a machine makes. And what about the listening device? Someone with compromised hearing abiliites - say a diminished perception of high frequency sound - might hear a machine differently.

One thing is certain: some machines do produce distinctive sounds, something that I'm more aware of when using a machine, but is more difficult to recall when not. I switch up the typewriter I'm using on a daily basis and will start paying more attention to their sounds and see if I can't contribute my own answers to those questions. In the meantime I'll be curious to see which models others will nominate for those categories.