I think the 100 nF to ground would filter high audio frequencies, but it's a bit outside my field to model the effect. I think one would need to know the impedance of the speaker. It's only in the circuit if the internal speakers are used, and those aren't exactly audiophile grade components, so it's probably not very significant.
A different connector would have been better, but I'm not so sure about adding just 10¢ to the BOM. I didn't find any 1/8" TRRS jacks with a DT switch on digi-key. I think such designs are no longer common. EDIT: Found *one* part, SJ-435107RS, that has an isolated ring switch. Digi-key doesn't have a parametric search for switch type, just number of switches. But CUI does, and this is their only TRRS switch that doesn't use ST switch connected to the signal pins.
The built in RS232 to USB bridge doesn't work how you think. It uses the native serial port, but allows connection through a standard usb connector with a standard usb cable. Instead of needing a specialized cable, with a built in rs232 to usb converter, and no standardized connector. It allows the rs232 voltage levels to be set in the board design instead of being determined by the cable. There is no interaction with the Linux USB system. You're probably thinking about a Linux serial gadget on a device or dual mode USB port, which is also useful, but not the same thing.
A different connector would have been better, but I'm not so sure about adding just 10¢ to the BOM. I didn't find any 1/8" TRRS jacks with a DT switch on digi-key. I think such designs are no longer common. EDIT: Found *one* part, SJ-435107RS, that has an isolated ring switch. Digi-key doesn't have a parametric search for switch type, just number of switches. But CUI does, and this is their only TRRS switch that doesn't use ST switch connected to the signal pins.
The built in RS232 to USB bridge doesn't work how you think. It uses the native serial port, but allows connection through a standard usb connector with a standard usb cable. Instead of needing a specialized cable, with a built in rs232 to usb converter, and no standardized connector. It allows the rs232 voltage levels to be set in the board design instead of being determined by the cable. There is no interaction with the Linux USB system. You're probably thinking about a Linux serial gadget on a device or dual mode USB port, which is also useful, but not the same thing.