Purism devices have hardware kill switches for the microphone/camera. How come there is no kill switch for the loudspeaker? As I understand, a microphone is basically the same as a speaker (source: https://security.stackexchange.com/questions/154343/can-a-speaker-be-used-as-a-microphone)
How does Purism prevent an attacker from using the speaker as a microphone when I switch off the actual mic?
For this attack you must have sound card, which can change audio output to audio input and passive speakers. Some soundcards have undocumented way to switch. But if Purism used discrete amplifier for speakers, then you safe, because any speaker with amplifier is active speakers.
Anyway, better to use system, which you can trust. You have gigantic choose: BSD systems, many GNU/Linux distributions, and you have great hardware support on this systems if you use Purism products.