I have a couple different KVMs. Neither of them have audio, but so far I've really not had any need for a KVM with audio.
On my vintage computer desk, I have an old four-port Aten CS-104 that has PS/2 and AT for the keyboard, plus PS/2 and serial for the mouse. I got it from the computer recycling place about 15 years ago. It doesn't combine the PS/2 and serial mouse signals, but it DOES combine the PS/2 and AT keyboard connectors internally, which makes it handy to use my AT Model M keyboard with the newer vintage systems that don't have an AT connector.
On my computer repair workbench, I have a ~2004 Linksys two-port KVM that's purely PS/2 only, someone gave it to me about five years ago. It's kind of annoying to use though, because it is self-powered and has a fairly long delay between when you turn on the computer and when the KVM becomes active, making it unfortunately common to miss short "Press [x] to enter setup" windows. It also seems to randomly mess with the left CTRL key on the keyboard I have connected to it, perhaps because that's part of a keyboard command sequence to switch inputs.